Extensions are plug-ins that add new functionality to Firefox. I recommend the following extensions:
These settings are for things like the user interface, etc.
Activate spell-checker in all fields
This will have spell-checking in single line textboxes as well as multi-line textareas.
Type about:config in the address bar. Type spellcheckDefault in the filter bar. Double-click on layout.spellcheckDefault. Change the value to 2.
Customize double-click to select
These are just two settings that control what happens when you double click on a word to select it. These are probably not useful to anyone else, but I like to have control over things such as this.
Type about:config in the address bar. Type layout.word_select.eat_space_to_next_word in the filter bar. Double click on layout.word_select.eat_space_to_next_word and change it to false. Type layout.word_select.stop_at_punctuation in the filter bar. Double click on layout.word_select.stop_at_punctuation and change it to true.
Cut and paste more than one line
Allow input boxes to accept multiple lines instead of just a single line when using cut and paste.
Type about:config in the address bar. Type singleline in the filter bar. Double-click on editor.singleLine.pasteNewlines. Change the value to 2.
Stop Firefox from automatically resizing images
I hate it when Firefox automatically makes an image smaller so it fits into the window without scrolling.
Type about:config in the address bar. Type browser.enable_automatic_image_resizing into the filter bar. Double click on browser.enable_automatic_image_resizing to change the value to false.
Disable link prefetching
Link Prefetching is where Firefox will try to pull down the next logical page you will visit as instructed by the web server. Servers can specify <link rel="next" href="Page2.html"> and Firefox will download that page in the background while you are on Page1.html in anticipation of you going there. While this will shorten the time you have to wait when you click on the Page2.html link, it eats up bandwidth on pages you may never visit.
Type about:config in the address bar. Type network.prefetch-next in the filter bar. Double click on network.prefetch-next and change it to false.
Want more tips? Check out some from the Firefox team.