This has been an ongoing problem on last several updates and definitely intermittent ,but seems worse in last two updates.Mostly when opening new site locations.
The Reset Firefox feature can fix many issues by restoring Firefox to its factory default state while saving your essential information. Note: This will cause you to lose any Extensions, Open websites, and some Preferences.
To Reset Firefox do the following:
Further information can be found in the Refresh Firefox - reset add-ons and settings article.
Did this fix your problems? Please report back to us!
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).
I don't want to do factory reset as I don't think that's the problem ,as it is so inconsistent as to when it acts up safe mode would be an exercise in futility.
Which extensions do you have installed?
Do you keep a large history?
There may also be problems with the sessionstore.js file that stores session data.
See Help > Troubleshooting Information
You can also create a new profile as a test to check if your current profile is causing the problems.
That will allow you to revert to the original profile if it didn't help or you are able to identify the cause.
See "Creating a profile":
If the new profile works then you can transfer some files from an existing profile to the new profile, but be careful not to copy corrupted files.