If you are having issues opening Firefox on a Linux computer, it may be due to a lock file in your Mozilla profile. This is especially relevant if you receive an error message similar to "Firefox is already running, but is not responding." This type of error can occur if Firefox is closed improperly or crashes, so the lock file remains, making Firefox think it's already running.
To fix this issue, you can remove the file from the .mozilla folder in your home directory. The file is typically located at a path similar to:
~/.mozilla/firefox/random_string.default/lock
If this does not work or you cannot locate the lock file, removing the entire .mozilla directory will reset all Firefox settings, and allow it to run in the default configuration:
rm -rf ~/.mozilla
Note that running this command will remove any saved bookmarks in Firefox.
For a further description of this issue, refer to the Mozilla Support article.