Linux, Firefox start problems

If firefox reports

Firefox is already running, but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system.

you can try to

close existing firefox windows, if visible
kill your own firefox processes using: pkill firefox
or ksysguard from a shell or KSysGuard from the System Tools menu.

If this does not help, remove the files

.mozilla/firefox/*/lock .mozilla/firefox/*/.parentlock

where /*/ refers to the profile names, usually only just one, eg

.mozilla/firefox/cqjv8dku.default/lock .mozilla/firefox/cqjv8dku.default/.parentlock

See alse the concerning mozillaZine page