How to Fix Firefox when it freezes completely after open it?

It sounds possible you have a problem with an Add-On causing an issue.

I would suggest attempting to start firefox in safe mode with all your addons etc disabled and see if that stops the application freezing on startup. You can do so by running the following command.

firefox -safe-mode

You should then get a dialog box with several options make sure that Disabled all add-ons is checked and click on Continue in Safe Mode. There are other options there to reset certain settings and features that you can also try if it turns out not to be an Add-On conflict. If that resolves the issue remember safe mode only applies for this session so you will now want to resolve which add-on is causing firefox to freeze.

First while in safe mode go to Tools > Add-Ons and disable all installed Add-Ons there that will make firefox run with no Add-Ons next time it is started in normal mode. At that point close firefox down and start it in normal mode to ensure it still runs without freezing and that all Add-Ons are indeed disabled.

Your next course of action if this is all working still is to attempt to enable each Add-On you have in turn, restarting firefox in between each to identify the Add-On which is causing firefox to freeze note it could also be a combination of Add-Ons so in the event that enabling only one at a time does not cause the symptoms next try these steps.

After running through every Add-On singly start enabling all your Add-Ons again restarting with each you enable until you produce the freeze, at that point you know that the last one you activated is conflicting with one of the others enabled prior. You can then use the safe mode trick to disable the others and try each of those candiates combined with just the last suspect Add-On you enabled to find the responsible pair of Add-Ons.

The cause was indeed a specific add-on. In my case it was an add-on called MEGA which supported downloads from the MEGA cloud drive service. I suspect the reason is the change of the URL.