Does anyone have any *general* suggestions about what I should try to look for or change in Firefox to make it work on sites which other browsers work on? (By "general" I mean "until I have a specific website to ask about".)
Restart using Add-Ons Disabled.
All the problems people face are because of Add-Ons messing around with Firefox's defaults. Defaults are there for a reason and the main reason is that it has been tested by users in the wild before it was released. then comes users like yourself who will start installing Add-Ons because somebody advised you to use them to enrich your life. The result is what you are currently facing!.
With over 1.3 billion devices now running Windows 10/11, customer satisfaction is higher than any previous version of windows.