The best way to fix the slowness or crashing errors in Firefox is to clean install the software. Here's a brief look at the steps to do a clean install of Firefox:
• Step 1: Open the settings on your device and uninstall Firefox from your PC.
• Step 2: Go to ‘Downloads’ and delete the Firefox installation folder.
• Step 3: Click on the C drive and open the Mozilla Firefox Program Files
• Step 4: Delete all the files and download a fresh copy of Firefox.
• Step 5: Double-click on the Mozilla Firefox downloaded file and start the installation process.
In case the problem continues even after you clean install the software you can call the mozilla firefox customer care number and ask for assistance.

For more info click here: mozilla firefox customer service | mozilla firefox technical support | mozilla firefox tech support