One of the quickest way to fix Mozilla Firefox performance issues like slowness or some broken add-on is by resetting it to a default state.
Here is how to do that :
1. Open Firefox and select Help > Troubleshooting Information (or type about:support in the address bar).
2. Click the button “Reset Firefox”.
What this will do is reset Firefox to default state while keeping cookies, passwords, browsing history and bookmarks intact. It will however not preserve existing themes or site specific preferences.
Pretty easy to fix and a time saver too.