By this time, Firefox 4 is no longer available in the MEPIS repo. I think MEPIS should put it back in, because I've had trouble (crashing, freezing) with the newer versions, especially 9 (I didn't know 11 existed, because I didn't see it in the MEPIS repo), and 4 works better. It's stable. I just downgraded to 7 for the moment, but am going to download 4. Although I can get it from Firefox, I think it should stay in the MEPIS repository, because apt-get installing is quicker than downloading from the Mozilla website.

Firefox 4 is end of life, it doesn't get any security updates or bugfixes from the Mozilla team. If the Mepis developers would take over the support for Firefox 4 they would have to re-brand it like Debian did with Iceweasel. If they don't support it it doesn't make sense to keep it in their repos.

using an old version of a browser is never a good idea . If anything you should be using Firefox 11 which works just fine.

With the trouble I have with Firefox 9, using an even newer version is the last thing I would do.

99% of problems are usually caused by incompatible extensions or themes. Or by using older versions of plugins like Flash or Java.

If running in Safe-Mode the browser doesn't crash then you can try eliminating extensions/themes one at a time until it stops crashing.

Other than bad add-ons there is no reason a newer version should crash, the whole reason behind newer versions is to fix bugs and security issues. Using an older version leaves you wide open for many security bugs/holes.

Upgrade and figure out what is making it crash.