CTRL+F in firefox – Super User

I’m having an issue with CTRL+F (the find feature) in Firefox. Mostly it works fine, but on some websites (for example this one), it just doesn’t work at all (the find bar comes up, but when I type words into it that exist on the page, it doesn’t locate them for me). When I restart Firefox in troubleshoot mode it works, which made me think it might be an issue with an extension, but when I disable my extensions one-by-one it doesn’t seem to fix it. What else might be causing this issue?

I’m (now) using Firefox 89.0 (64-bit) for Ubuntu, but this issue has existed for a long time.