Do you want to try Application settings:

Settings app > Apps > Firefox

If the FORCE STOP button is available, I would try that first.

Then if the UNINSTALL button is available, I would try that next.

Does it work?

Doesn't work. Phone reboot when I press the uninstall button

Oh, maybe you already tried that.

I think where programs are installed might be a protected location that you can't erase using a file manager.

Perhaps if you sideload Firefox from a different site then it might be able to overwrite the problem program folders. Then you could try to uninstall it again. I actually don't know for sure which of the folders on the following site are for which Androids, but I think API 9 is for Gingerbread and API 11 is for newer versions of Android (and x86 is for Intel chips, so that doesn't seem relevant at all):

http://ftp.mozilla.org/pub/mozilla.org/mobile/releases/latest/

My mistake, I forgot to mention that it's the Firefox beta that I'm not able to remove from my phone.

Can you give me access to the .apk for the beta version?

timononk said

Can you give me access to the .apk for the beta version?

It might be in this folder, but I'm not sure:

http://ftp.mozilla.org/pub/mozilla.org/mobile/candidates/