[CentOS] How do downgrade mozilla ?

Hello

Sorry to say this to everyone, but since I installed CentOS 6 a month ago 
I found that both Mozilla Firefox and Mozilla Thunderbird crash big time 
(meaning every day and even more than once a day) with a large INBOX or 
newsgroup, or with many tabs open in the Firefox windows. I also keep my 
work computer running over night usually, maybe this is also a factor.

Knode does not do a better job than Thunderbird with the newsgroups, 
either, it still crashes on large ones. Konqueror too consumes many 
gigabytes and becomes unresponsive if I have many tabs open and keep it 
running over night. I got pan newsreader though, and it is very nice with 
a 126000-articles a newsgroup (like on CentOS.general :)!

And that reporting tool that pops up every time says it can not upload 
the crash report (because it does not have some valid URL or something)...

Is there a nice way to downgrade Mozilla Firefox to say version 4 or 5 
please ? And something similar for Thunderbird ?

Thank you,
Timothy Madden