I was an IE user for a decade or more but have now been using FireFox exclusively for well over a year. 

Personally, I don't like the font rendering in FireFox - when viewing a page in IE, the font smoothing makes a real impact and sites look much better... but there are enough improvements over IE for me that I can live with this issue.

FireFox has a couple of great features that are the driving force for me to have made a permanent switch in browser:

1. Error Console - this has been an invaluable feature to me for debugging issues with JavaScript.
2. Extensions - MS missed the boat with IE by trying to force their plug-in technology around ActiveX.  FireFox extensions are simpler.  They are small software programs that literally extend the function of FireFox.  There are hundreds of them created for all sorts of reasons.. and just about all of them are free. 

From an SEO point of view, there are several that are really useful.  And so I thought I'd share the current set of extensions that are installed on my PC.

1. IETab.  Yes I know it is not an SEO extension - but it is incredibly useful.  If you need to be able to see how a site is rendered in both IE and FireFox, then you can installIETab and switch between the two from within FireFox (you view a web site via IE .. but embedded within FireFox).  Naturally this useful for comparing sites, but some sites are IE only which makes this extension essential (e.g. Microsoft's Outlook Web Access).

2. SearchStatus.  This extension provides a number of functions that you can perform on a site. 

• You can highlight nofollow links - useful to quickly check which of the links on a page have the "rel=nofollow" tag.
• Provides a link report - counts how many links are external, internal, etc
• Meta tags - allows you to quickly see the meta tag info without having to view source
• A variety of look up features - whois, archive.org etc
• View the robots.txt file for the site
• Keyword density tool
• Search engine reports that show how many links are being reported by the various major search engines

3. SEO for FireFox.  This extension modifies the search results that are returned by Google (etc) and allows you to lookup additional information next to each result - e.g. the pagerank of the site, the age of the site, number of links, etc.   I disable/enable this as I need it - as it can make the results look complicated when I am searching normally.

4. Total Validator.  This is a nice tool that will load a site at the click of a button and provide w3c validation, spell checking and broken links checking - all from the same tool -- very nice.

5. YSlow.  I use this one a little less, but it provides load time stats which are increasingly becoming important (especially with Adwords).

Well that's a list of my extensions - there are many more available that are useful for search engine marketers, but this list will provide you with a solid base.