I am a web developer. It is more precise to say that I am a front-end web developer. This means that I need developer console each and every day.
For years now Chrome's development tools have been much much better for me as a developer than what is available in Firefox. So I use Chrome (mostly) for development.
However I also have personal needs when browsing as well as professional, not related to coding. In this case I love what Firefox has to offer.
Here are reasons why I prefer Firefox over Chrome for my browsing:
Unlimited tabs open without noticeable slow down. This is important for me, especially when I read my news feed, I tend to open all interesting articles and read them after I am done with the list. Often this means 30+ tabs.
Configurable tab opening behavior. Chrome was not able or not willing to implement that for years: I want my tabs to open in the background without me having to use the middle mouse button. Why? Well because most products (apps) use shortcuts there days and I love them. Even in Google Plus you can press "v" to open the linked article and in Firefox it goes to load in the background while in Chrome the browser switches to the newly opened tab and I have to wait and stare first at the blank page and after a while at the half rendered page and after that maybe read the article, but usually switch back to the original opener tab. There are plugins that try to handle that, but none of them works correctly.
Much faster start up. One thing Firefox learned to do is to load only the pinned tabs and the one that is focused when restoring a browsing session. This means that I can close my browser at any time and go back to it as it was without having to wait like 5 minutes for it to become responsive. On top of that even without many tabs Chrome starts much slower, on a few years old computer (core 2 duo with 4 gigs of ram) Chrome starts too slow (mainly because it tries to read so many things from the disk I think). Once it starts it is okay, but this means I have to keep it preloaded all the time for it to be as responsive on start up as Firefox. No thanks.
Omnibox completion. This one suck big time in Chrome. At the beginning Firefox used two inputs - one for the address (address bar) and one for the search (search bar). However for many years now one can just remove the search bar and use the address bar to search Google (or any other set as default search engine address). On top of that it uses your bookmarks, your history and your recent searches and it learns to be much more of help than what Chrome does. Chrome for example almost never uses my bookmarks to help my with the typing, it is limited to only 5 entries and usually most of them are Google suggestions. On top of that there is no easy way (panel or otherwise) to search your bookmarks quickly. Chrome has those "apps" installed that are just links, but they are not configurable. For example I use InoReader, but their 'app' icon uses http. I want to use https for the service, but even if I add it as a bookmark Chrome never completes it for me. I have to navigate my bookmarks and find it. Way to frustrating.
Pinned tabs. In Firefox you cannot accidentally close a pinned tab. In Chrome pinned tabs are not really pinned, they are just visually reduced to be as large as the faveicon.
Expose like functionality for sessions. Okay this is really interesting. With this one Firefox allows me to have several filled with tabs views into the same browser window. I had 3 and kept them there for months before I had the time to read all of them and the browser remembered them. In chrome I am forces to bookmark those and mark them as "read it later". But those should not really be bookmarks. Note that this cannot be simulated with several Chrome windows, because I cannot close all windows at once easily.
Often used web sites: Chrome and Firefox have the ability to display thumbnails of your most often visited web sites. What Firefox allows you to do on top of that is to pin your favorites and thus have quick access to the ones you think are most useful and not the ones you really visit most often. This is to say the tool is versatile and you can use it as you like, not as someone has decided for you.
Superior plugins. At this point I have only one in mind but it works much much much better in Firefox than anything close to functionality in Chrome - ScrapBook. In Firefox the extension uses nice side panel, has search and most importantly - saves your documents at configurable location. For example in your Dropbox folder. Chrome variants use indexedDB and cannot sync between several computer in no usable way. And this tool has proven to be so useful that I simply cannot forgo it.
Better support for themes. You can make Firefox look like Chrome. You cannot make Chrome look like Firefox.
You can force sync. In Firefox the sync button is exposed and you can force the sync. In Chrome if you make a change right before you close the browser it might not be populated to the server. This is especially bad in relation to the fact that a bookmark has to be used to mark a web page for later reading (see above).
Last tab closed behavior: In Firefox when you close a tab and it happen to be your last tab the browser does not exit, instead a new tab replaces it. This is also configurable. The problem is that I am just closing a page, I do not have to pay attention if it is the last one in the window. This is very irritating especially when combined with the slow "fresh" start of the Chrome (see above).
AdBlock(+): This almost never work as intended on Chrome. Maybe it is developers problem, not necessarily a browser problem, but still as an end user it irritates me.
If I think hard enough I can come up with several other reasons, but I think this suffice to say that I am a bit on the 'advanced' user side. Chrome appeals much more to those that do not rely so much on their browser to comfort their browsing for whole days/nights. I know people that use it very happily, do not have even a single bookmark and most often do not even know which browser they are using. This is all fine as well and I know Firefox is not for everyone, neither is Chrome
What I miss from Chrome when using Firefox?
As much as I love Firefox there are aspects of Chrome that I like and miss when using Firefox. A list follows that describes the most missed features.
Open as window app. Using the 'app' icon you can configure a URL to open in a new window as a separate app. It is especially useful for apps that do not use popup windows and are examples of the so called fat clients or rich apps: Gmail, InoReader, Youtube (especially now that it does not reload the page) etc. The window manager picks up the face icon as application icon and you have somewhat more real application on your desktop.
Task manager: Sometimes while browsing I notice that my fan goes on and on at the highest speed. Chrome allows me to see the CPU.Mem usage of each separate tab. Well, this is not entirely true, it shows me tabs grouped by processes, which is not the same as being per tab but never the less it allows me to narrow down the offender. On top of that it can give useful information from developer's point of view.
Offline apps with escalated privileges. I do not use one in particular that cannot be implemented well with IndexedDB and AppCache (that is to require access to hardware outside of the standard web APIs) but still it is good to know that it is there if you need it.
Chrome makes features available faster than Firefox. Such example is Speech synthesis and recognition, File API etc, things Firefox users have to wait for much longer.
Chrome has more horse power when it comes to raw JS performance: For most apps JS runs smooth on both browsers, but computation intensive apps tend to be more responsive in Chrome. One such example is Google spreadsheet. I have spreadsheets with thousands of records and tens of sheets and thousands of formulas and those run around 20% faster on Chrome. I am not sure if it is because off the browser or because they were fine tuned for Chrome but it is the fact.
This is again not all, but what comes at the top of my head, the features I miss the most on my day to day browsing.
I am really pro-Firefox for several ideological reasons as well, but I am not too optimistic for its future. They as a company have dedicated too much effort in Firefox OS, a project that is born dead in my eyes. On the other hand Google uses Chrome to make it possible to create apps that run fully offline on the desktop and on the mobile (packaged apps) and eventually this will make them a leader as a target platform for development while Firefox will remain marginal "proof of concept" platform with no adoption outside the geek community and is essentially waste of time and money. I know that Mozilla is non-profit, but making useful things for the users is IMO more productive than to make a very limited-use OS.