Firefox has been giving me a few problems lately, but not the same as you describe. Assuming you have a few add-ons installed, it might be worth starting Firefox in safe mode. To do so, click on the "Open Menu" three bar icon in top right, then click the question mark and finally "restart with add-ons disabled". If that cures it, then you can re-enable your add-ons one by one to find which, if any, is the culprit.
If safe mode makes no difference, then I must leave it for others to resolve.