If you are on Windows:

1. Press Windows + R keys to open the run panel
2. Enter firefox.exe -ProfileManager and click on OK
3. Then create a new profile

For all systems, try to start Firefox in Safe Mode by holding down the shift key of your keyboard.

After you create a new "blank" profile using the Profile Manager, as described in Banban's reply, we will need a little more information to help you recover your previous data. In particular:

Did you start using the Firefox Refresh feature? Normally this would move your old settings folder to the desktop in an Old Firefox Data folder and generate a new profile folder. Some users have reported that their Refresh did not run to completion or seemed to have stopped working so they interrupted it. In this situation, we suggest working with the information in Old Firefox Data. For example (written for Windows 7): https://support.mozilla.org/questions/1069219#answer-762489

If you do not have an Old Firefox Data folder with your latest settings in it, we'll suggest how to explore for the folder elsewhere on your computer. Throughout this process, please COPY and do not MOVE files and definitely do not DELETE anything, so that if something goes wrong, you have a second chance to try again.