I'm not sure when I first started using Firefox but I haven't looked back. I still run into the occasional site that will work in IE but they are few and far between. One of the things that makes Firefox such a good product is it's extendability. Developers can add additional functionality to the product by creating extensions. Here are the extensions that I currently use:

FlashGot - This extension works with various download managers to make downloading easier and more manageable. I use the Free Download Manager and the integration will let me download one or all of the links on the page.

SpellBound - This one is great for anyone posting to forums or blogging. It allows you to left click on a text entry and check the spelling. You can be sure that I will use it before I save the final for this entry.

Mozilla Archive Format - Another must have extension. One of the things I liked about IE was the MHT file format allowing you to save a web page into a single file. The Mozilla Archive Format brings the ability to not only read the MHT files that you saved in IE but save those in the MAF format.

IE Tab - I've actually just started using this extension today but so far it works as advertised. It allows you to open a page in IE without leaving your favorite browser. Now you never have to open IE again.

Google ToolBar - Google finally got around to putting out a version of their toolbar for firefox.