I don’t really use twitter, but I’m obsessed with this problem I’m having getting it to work in firefox. I can’t unfollow or do some other functions because the javascript fails. What’s driving me crazy is I cant find any info about it either searching Twitter’s help pages, Firefox’s help pages, or the web. I put in a help request to Twitter, describing the javascript errors that I can see using the debug console. I get errors like “setting a property that has only a getter” and “twttr is undefined”. These seem like problems that would have simple fixes, but I can’t figure out why there aren’t hundreds of posts and bug reports about it.
That being said, I found a way to unfollow using the terminal command line in Linux. The solution is here: http://apiwiki.twitter.com/Twitter-REST-API-Method:-friendships%C2%A0destroy and several other places. For a test, I unfollowed the NYTImes with this line: curl -u bnmng:mypassword -d “screen_name=nytimes” http://twitter.com/friendships/destroy.xml
BnmnG 
I just installed Ubuntu this weekend, and I’m having the exact same problem. I set my computer up as a dual-boot system, so I can switch to EvilOS when I run into a problem, but it’s not the best solution. Hopefully someone somewhere figures it out.
I’m still amazed that so few people have posted about this problem. For me, disabling firebug worked.
Sorry for your trouble but glad to have some company. I’m surprised at how little info I can find about this problem. Maybe there just aren’t that many Ubuntu tweeters.