brotab for Firefox only works when the config directory is located at ~/.mozilla

[seanhly]

Reproduction step:

• Exit firefox if it is running.
• Move your firefox config directory from ~/.mozilla to ~/.config/mozilla
• Restart firefox.
• Brotab no longer works, and it looks as though bt_mediator isn't running in the background at all.

Additionally, if I softlink ~/.config/mozilla to ~/.mozilla, next time I open Firefox brotab magically works again.

[seanhly]

FYI, ~/.config/mozilla is now the default configuration location on many Linux firefox installs.