Firefox's Safe Mode actually disables a bunch of other settings besides extensions, including:

• JavaScript compilers designed to speed up execution of complex scripts (JavaScript is interpreted the old-fashioned way)
• Hardware acceleration of graphics and smoothing of text using your graphics card/chipset (falls back to using the CPU)
• Custom style rules in an optional userContent.css or userChrome.css (rules ignored)

Since you described the problem as being pervasive and not limited to complex sites, perhaps it's the graphics? Could you try disabling Firefox from using hardware acceleration? Here's how:

Windows: "3-bar" menu button (or Tools menu) > Options Mac: "3-bar" menu button (or Firefox menu) > Preferences

In the left column, click Advanced. On the right side, with the "General" mini-tab active, uncheck the box for "Use hardware acceleration when available".

This takes effect the next time you exit Firefox and start it up again. Any difference?

Since hardware acceleration improves the appearance of fonts and animations, you may want to check whether graphics card/chipset driver updates are available for your system. This article suggests safe ways to do that: Upgrade your graphics drivers to use hardware acceleration and WebGL.

Thanks! That's smart, but I've actually already disabled hardware acceleration. And yes, the problem is pervasive—spinning disc and cursors that won't appear on every page I try to open. Do you have an idea what I could check next?

Do not advocate for any Extensions but there is one might find useful or not. Fat-Free Fox Let us know please if any changes.

Note that you use a Firefox beta version (make sure to update to 55.0beta4).

Does this also happen with the regular release version?

Note: I recently read a warning about downgrading from Firefox 55 to an earlier version:

Breaking profile changes - do not downgrade Firefox and use a profile that has been opened with Firefox 55+.

https://www.mozilla.org/firefox/55.0beta/releasenotes/

I haven't seen any documentation on the smoothest way to handle this. You may need to create a new profile after downgrading. Hopefully someone will clarify soon.

Maybe this bug about addonStartup.json.lz4

• Bug 1358846 - Merge extensions.ini, extensions.xpiState, extensions.bootstrappedAddons, and extensions.enabledAddons into a single JSON file

Thanks for your help, Cor-el! I've tried downloading the stable version, and the same problem happens when I use that. You might be right about the Bug 1358846, but how do I solve it?