You can try something like User Agent Switcher.
I'm a little bit confused, how would switching my user agent for MediaFire allow me to access MediaFire's MyFiles's page?
If it thinks you're using a newer browser, then it won't show that page. But I'm guessing they wouldn't just block a version for no reason, so it may well be that there is some bug or incompatibility which stops that page from working in firefox 3.
Thanks for the explanation

, very helpful

!
Honestly, you should just upgrade to a newer version of Firefox - there's really no benefit at all to sticking with such an outdated browser, and if it's stopping some websites from working, then that's all the more reason to fix it. What's so bad about newer versions of FF, anyway?
My main problems with the newer firefox are the following.
1.) The Status Bar at the bottom. I can't make it look like it was in Firefox 3.6.24.
2.) When I right click on a link, Open Link in New Tab is above Open Link in New Window, instead of Open Link in New Window being above Open Link in New Tab.
3.) Selecting Tools -> Addons makes the add-ons window show up as a new tab, instead as a new window.
4.) Oh and yeah, some of my add-ons are incompatible.
That's all I can think of right now.