That shouldn't happen...

Is it possible you used the Refresh feature during your update? That could happen if Firefox says it is performing slowly and you can speed it up by restoring some default settings. If you use that button, and Old Firefox Data folder appears on your desktop. Can you find anything like that?

If so, great, we'll look in there for your passwords. (Details to be provided if applicable.)

If not, have you noticed any other changes in Firefox after the update?

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

You would need a (backup) copy of the logins.json and key3.db files to recover the passwords if you current copies are lost or corrupted.

• Tools > Options > Security: Passwords: "Saved Passwords" > "Show Passwords"

• https://support.mozilla.org/kb/Recovering+important+data+from+an+old+profile
• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox