Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac Options) key, and then starting Firefox. Is the problem still there?

In "Safe Mode" the "Hang-freeze" does not occur. Reverting to normal use, after the "Home Page" (which in my case is Google), I opened Facebook - the Display and onscreen clock froze for 20 seconds...then the clock "caught up" and everything runs fine. It really is not much of a "problem", just a quirky thing - I moved to Firefox from IE8 years ago because IE became a monstrous beast, slow to load, crashing, running slow, etc., and now, although Firefox is, after 4/5 years, also becoming a great beast of a program, this is the first, and only, oddity. I merely report it because I am truthfully puzzled.

Okay, let's start with;

Type about:preferences#advanced<Enter> in the address bar.

Under Advanced, Select General. Look for and turn off Use Hardware Acceleration.

Restart Firefox. Poke around safe web sites. Are there any problems?

It could be the work of one of your add-ons. Type about:addons<enter> in the address bar to open your Add-ons Manager. Hot key; <Control>(Mac:<Command>)<Shift> A)

In the Add-ons Manager, on the left, select Extensions. Disable a few add-ons, then Restart Firefox.

Some added toolbar and anti-virus add-ons are known to cause Firefox issues. Disable All of them.

If the problem continues, disable some more (restarting FF). Continue until the problem is gone. After, you know what group is causing the issue. Re-enable the last group ONE AT A TIME (restarting FF) until the problem returns.

Once you think you found the problem, disable that and re-enable all the others, then restart again. Let us know who the suspect is detective.