Sorry to hear it is not working so well for you.
I don't work for StackExchange and cannot give you an official answer, but I'll share my impression of one reason why you might not want to do this rendering on the server. Doing it on the server side would degrade the quality of some site features. For instance, when users type in an answer (or edit an answer), they wouldn't be able to get real-time feedback seeing how the answer looks. That's an important and useful feature for many users. Breaking that would be most unfortunate.
This site is not optimized for folks with older, slower computers, so yes, if you have an older, slower computer, the site will be slow on questions that use LaTeX equations. That's probably a fact of life at this point. I doubt StackExchange will do anything about it -- nor would I recommend that they change anything, as the current set-up seems to work well enough for the overwhelming majority of the community. It's not reasonable to expect them to make significant changes to support one user, so unless there is an enormous groundswell of support from this community for making such a change, I doubt any such change is likely to happen.
Firefox 3 has major security issues, which can be exploited by just visiting an (infected) website. By using software with known security issues, you might be liable for damages caused by attackers using your computer as a relay.