Back and forward buttons invisible on FFox v90 and up - back and forward buttons don’t show on v91, on v90x they show for a second if you roll over, find them and click - ok on other browsers

Do you mean the angle bracket markers on the left and right in the middle of the page? I see them with Firefox release 90 and Nightly 92.

Try Troubleshooting mode to see if an add-on is causing the problems and try in a new profile to see if there is some other problem with your profile.

Mac High Sierra 10.13.6 Firefox Nightly 92.0a1 (2021-08-02)
brand new profile, not even signed in, no addons

Screen Shot 2021-08-02 at 05.22.43 PM
forward and back buttons invisible - will advance the page if I roll my mouse over to find the spot- some versions of FF show them if I click around. This sshot shows the url at the bottom left where the cursor is placed on the invisible left arrow - unfortunately the screenshot does not show the cursor itself.
TV Guide - pulldown under the time 2 or 3 hours should have red indicator