Could you check to make sure the "caret browsing" feature is not enabled? You can check that here:

• Windows: "3-bar" menu button (or Tools menu) > Options
• Mac: "3-bar" menu button (or Firefox menu) > Preferences
• Linux: "3-bar" menu button (or Edit menu) > Preferences
• Any system: type or paste about:preferences into the address bar and press Enter/Return to load it

In the search box at the top of the page, type cursor and Firefox should filter to the "Browsing" section. If the box is checked for "Always use the cursor keys to navigate within pages" then uncheck that box.

If you changed that, does it make any difference?

More generally, when there are two different scrolling regions in a page, it sometimes helps to click in the region that you want the cursor keys to scroll. Does that make any difference?

Unfortunately, caret browsing does not seem to be the issue. I just looked and the "Always use the cursor keys to navigate within pages" button was always unchecked and testing it again showed no new results.

I have tried clicking in various regions of the page, but it didn't seem to do anything either. I'll put a screenshot of where I tried clicking in case I missed something.

Alright, so I was playing around with a few more settings and found out where the issue was. Apparently, jumping to a new section from the sidebar changes the zoom setting to "Page Fit," which is where the arrow keys start acting up for some reason. Changing the zoom setting to something like "Page Width" fixed the issue.