AdBlock plus: this one should be installed by default with every Firefox installation IMO. It not only blocks ads, it also does good job with unwanted flash ads, which at this point in my browsing habits makes flash block unneeded. However FlashBlock is also great.
Fast dial: One feature I miss from Chrome is the 'app' shortcuts (i.e. not really applications with off-line cache and stuff, but instead simple bookmarks to a URL on the web. Fast dial goes close enough, it is not that pretty (i.e. it does not display the supplied icon, instead it makes a screen grab of the web page by default, which might be a plus or a minus. For example I have had 3 apps in chrome that used the same google reader icon, which is confusing. On the other hand the icons look much nicer.) but does essentially the same thing. And you do not need to go to a web store. It comes very handy when you have multiple very similar bookmarks (like the example above with google reader and google reader player). Fast dial supports icons also, but you need to provide the URL for the icon manually.
Pixlr grabber: Pixlr is a great image editor (much better than the one used by Google Images (aka Picassa). This add-on lets you grab part of any page and edit it in pixlr with one click. Similar to Awesome Screenshot for Chrome and Firefox, but provides much better editing tools. On the other hand if you simple need to make annotations (like pointing out rendering issues to your web designer) awesome screenshot is the better choice.
Quick translator: I use 6 human languages day to day, however I am not so good at all of them and sometimes I need a bit of help with a word or a phrase. Instead of opening new tab with google translate and copy/paste the word I use this tool. Also notice to Chrome - instead of offering to translate each page allow the user to select a text to be translated - less intrusive. Besides google translate is their product also.
ScrapBook: this one is probably the most useful for me, as it allows me to do most work. I use dropbox folder (actually encrypted dropbox folder but this is entirely different story) to synchronize my scrapbook across my computers, it would have been nice to have another option for this, for example using google account or other service, but dropbox is just fine for now, providing they do not loose my files. What I like about scrap book is that it allows me to annotate the saved pieces of document, it allows me to keep an entire document, it keeps link to the original URL so I can visit and check for updates (like on forums or blogs) and I have a copy of the content locally, so even if it disappears from the web I can still access it. It allows for folders, exports and other nifty tricks and it more useful than anything I have found thus far. For once I do not need to keep bookmarks to developer sites just to check 3 line of codes I keep forgetting. I just save the parts I like and I can go back to the URL whenever I need to check for updates.
I also use firebug a lot, but this is related too my job. I also use greasemonkey but I write my own scripts, I rarely even search for scripts for domains. Probably I am just happy with what I use as web applications right now.