Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

Thanks for the suggestion, FredMcD. Starting Firefox on safe mode didn't fix the issue. I am posting images comparing the same website (delta.com) in FireFox and in Chrome (I'm on Mac OS X High Sierra, updated yesterday (10.13.1)). It does happen on few other websites too, on FireFox and not Chrome (or Safari).

It looks to me like, for some reason, the scrollbar thumb on that site in Firefox is the same color as the scrollbar track, rendering the thumb invisible. And that is not the case in Chrome.

I'm an avid FireFox user, and would prefer to keep on using it. Any way to change these colors as an end user? I do use the scrollbar enough (and Delta!) to be impacted by this issue.

Any help is and would be highly appreciated, thank you so much!

Hmm. There are add-ons you can get that alter the scrollbar. You can also click the middle mouse button changing the mouse into a scroll controller.

Go to the Mozilla Add-ons Web Page {web Link} (There’s a lot of good stuff here) and search for what you want.

Thank you, FredMcD. It's a strange (and sudden) problem. I'll try your suggested options later today/tonight and will post the result tomorrow. Thanks again!

Could be a bug on High Sierra.

Pages with a dark background will fail to show a scrollbar thumb if scrollbars are set to always display.

• bug 1408701 - Firefox scrollbar has no thumb on some pages (on macOS 10.13?)

(please do not comment in bug reports
https://bugzilla.mozilla.org/page.cgi?id=etiquette.html)

Looks that it is fixed in the current 57.0beta.

• https://www.mozilla.org/en-US/firefox/beta/all/
• https://www.mozilla.org/en-US/firefox/beta/

Thanks Cor-el! The latest beta version, as you suggested, does seem to solve the issue. So, I'll just wait for the next full version of FireFox which, I think, is due out next week.