Clicking a button within a webpage puts a blinking text cursor in the body of the site, as if editing a Word document. It’s not causing any other problems besides being really annoying and distracting, but it only just started today. I’ve tried restarting Firefox with add-ons disabled, as well as reinstalling Firefox clean.
Fortunately the solution turns out to be pretty simple:
This is likely caused by switching on caret browsing and you can toggle caret browsing on/off by pressing F7 (Mac: fn + F7).