use

How do I start Firefox or Thunderbird when it is stating that my 'profile is in use'?

With Netscape:

rm $HOME/.netscape/lock

or

rm ~/.netscape/lock

For Mozilla:

rm $HOME/.mozilla/*/*/lock

or

rm ~/.mozilla/*/*/lock

For Firefox:

rm $HOME/.mozilla/firefox/*/lock
rm $HOME/.mozilla/firefox/*/.parentlock

or

rm ~/.mozilla/firefox/*/lock
rm ~/.mozilla/firefox/*/.parentlock

For Thunderbird:

rm $HOME/.thunderbird/*/lock
rm $HOME/.thunderbird/*/.parentlock

or