I haven't experienced this issue myself on my Mac. I just visited facebook.com on a brand new tab and as well as a tab that I was working on previously. In both the circumstances, I was able to use the back button. Can you provide more details like the OS that you are using, the version of Firefox, and if possible a screenshot as well?

Hi, thank you for your reply. I'm using Firefox 76.0.1 (64 bit), and Windows 10 Pro 1909.

A screenshot wouldn't tell you anything except a greyed out back button as it would look like on a brand new tab. I did however see that the back button worked as it should on a private window.

When you go to Facebook, did you use a new tab or window?
In other words, was FB the first place you went?

Many site issues can be caused by corrupt cookies or cache.

Warning ! ! This will log you out of sites you're logged in to. You may also lose any settings for that website.

• Delete cookies to remove the information websites have stored on your computer {web link}

• How to clear the Firefox cache {web link}

If there is still a problem, Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Did this help?

While you are in safe mode;

Try disabling graphics hardware acceleration in Firefox. Since this feature was added to Firefox it has gradually improved but there are still a few glitches.

How to disable Hardware Acceleration {web link}