Solving mozilla problems
Sometimes, after not clear exit from mozilla firefox or mozilla thunderbird (e.g. after power outage) they can't start, resulting in error message like "Forefox is already running but is not responding". To fix this problem, you need to remove the lock file, namely for firefox
cd ~/.mozilla/firefox/<some_symbols> rm .parentlock
where <some_symbols> is the directory name which corresponds to your profile. For thunderbird
cd ~/.thunderbird/<some_symbols> rm .parentlock
After starting firefox, you may encounter another error: "The bookmarks and history system will not be functional". The exhaustive description of ways of solution can be founded at Mozilla support site. Shortly, close firefox, then
cd ~/.mozilla/firefox/<you_profile> mv places.sqlite places.sqlite.old mv places.sqlite-journal places.sqlite-journal.old
Then start firefox. Page history will be lost, bookmarks will be restored from the last backup.