I have had that before and I had to restart to solve the problem

I am having a similar problem as well now. It was working fine with no problems
until I installed an update (in Kubuntu). I am now using Opera and find it a
poor substitute for Firefox. I have rebooted several times but it still doesn't
work. I have even tried removing, purging and re-installing it to no avail. It
has now got to the stage that I can get it loaded up but it just turns itself
off with no error message if I try to go to any site other than the home page it
loads when I start the program.

I occasionally have a problem with Firefox.
If there is no evidence of it running go to a terminal and type ps -e
[Enter] to see all the processes.
If it shows up on the command line type killall firefox [Enter] (might
need firefox-bin or whatever the process is called)

From this you should be able to run Firefox, it might ask if you want
to restart the original session - and go from there.

May be your profile is corrupted. It does happen sometimes when firefox
abends. It could also be an addon that is in problem. In that case you have to
disable all addons and bring them back one by one until you find the problem.

Try to start firefox in the profile manager first and see if you can create a
new profile that works.

You have to start it from a shell with: firefox -ProfileManager&

If it does work with a new profile, then copy your bookmarks/cookies from the
old profile, delete it an set the new one as the default.

I have used the Software Sources option in the Admin menu to remove a
program, it seems easy and successful. I would then dowload firefox
again via Synpatic.

It is showing on another desktop, just try CNTL + TAB and you should
see it.
Set your Multiple Desktop settings to show all desktops on the bottom
bar, or to shpow only ONE desktop if you are not familiar with multiple
Desktops.