Start Firefox in Use Troubleshoot Mode in Firefox to check if one of the add-ons is causing the problem (switch to the DEFAULT theme: Tools > Add-ons > Themes).

• Don't make any changes on the Safe mode start window.

See:

• Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Worked in safe mode, then I restarted Firefox in regular mode and now working properly. Did not disable any of my add ons.

OK, I think I have found the issue. If you have imported your bookmark groups or have previously set them up as group that opened just from clicking on the group - it will not work.

I have created the group again and all works fine, I have another issue now - I can not deleted the old group from my list!

Hope this helps.