EDIT 4: I think one potential cause of issues is either normal tab discarding or Auto Tab Discard.

Either way, toggling disable -> enable of the add-on in the add-ons list fixes it.

EDIT 3: Bumped back up to 5 stars though because it's still so incredibly useful.

EDIT 2: Frequently stops working for some reason... Rules just randomly stop applying.

EDIT: Also would make my life a ton easier if it also could dynamically set the title based on the contents of a set text input box on a web page

Fantastic, especially regex rules, though I really wish I saw "Regex Error" appended a hell of a lot less often. (I think the most recent case I'm running into is maybe because the website uses a temporary different title during tab loading? I'm using $1 if that's important.)