Firefox Freezes due to Compositor

Symptom:

The Mozilla Firefox web browser has a variety of unpleasant symptoms. In this week's zombie invasion, these were seen, in approximate order:

• On the Fidelity Investments login page, you authenticate, and it says (in red): An error occurred while accessing this site. Try again later. Frantic call to customer support: did you guys get hacked? No we didn't get hacked, try updating your browser.

• Opening non-complicated pages becomes very slow.

• Opening pages becomes even slower. Firefox obviously downloaded the page because the window titlebar shows the page title, but nothing is rendered. Even weirder, if you click in an entry box (e.g. address bar) there is no visible effect. However, if you type blind and hit Enter, Firefox downloads the new page, changes the titlebar, and doesn't render the new page. Also, if the window loses focus (because you raise another window over it, or you iconify it), when it regains focus the titlebar typing becomes visible, and if there was a dialog box (You are about to close multiple tabs…) it becomes visible for the first time, and it obeys (quits) if you click in it.

• The Profile Management Widget has the same issues. (firefox -P) This is what you do to create a new profile.

• When you remove .mozilla and .cache, and start firefox, the situation is not improved.

What's happening:

I wish I knew.

A key item here: I'm using the XFCE desktop environment. None of this has been tried for comparison on Gnome or KDE.

I renamed my home directory and created a new empty one, and I went through a lot of tries at selectively restoring particular subsets of files. The finger of blame pointed at ~/.config/xfce4/xfconf/ , but attempts to restore some but not all of these configuration files frequently ended up choking xfce4-session so I couldn't log in.

I tried regenerating my XFCE configuration from scratch, having written down what all the non-default settings should be. One item jumped out at me: the compositor (in Window Manager Tweaks). It's off on my laptop; why? Because Firefox froze up several months ago, I thought of that as a likely troubleshooting interventions, it worked, and I failed to document it.

So Firefox is back in action on my desktop machine (and the laptop).

• Desktop: Intel NUC7I5BNH, CPU: Intel Core i5-7260U @2.2GHz; graphics: Intel Iris Plus Graphics 640; chipset: Sunrise Point-LP / Kaby Lake-U
• Laptop: Asus Vivobook S15 S532; CPU: Intel i5-10210U @1.6GHz; graphics: Intel UHD 620

How to fix:

In the Whisker Menu find the Settings group, and within that, Window Manager Tweaks. Flip to the Compositor tab. The first setting is Enable display compositing. Turn it off.

It appears to be using software compositing, since transparency effects are still seen (e.g. in notification pop-ups).