If you face errors or problems (crashes, slow speed, etc.) while you are using your Firefox browser, then you can reset its settings in order to bypass these errors. This means that all your settings (homepage, extensions, search provider) will be reverted to their default state (as in a fresh installation).
How to reset Firefox browser settings to default.
1. From Firefox menu, go to “Help” > “Troubleshooting Information”.
2. In “Troubleshooting Information” window, press “Reset Firefox” button to reset Firefox to its default state.
3. Press “Reset Firefox: again.
4. After the resetting job is completed, Firefox restarts.
Now see if your browser works smoothly, otherwise check your computer for malware infection by using this guide: