On reflection, it’s also quite possible this is a platform-specific bug. I’m on Linux. I think I experienced it in Windows a few years back, but the memory is fuzzy.
… hmm, apparently if I start a blank Firefox profile this doesn’t happen either. Though I know I’ve encountered this with at least two completely distinct Firefox profiles over the years. This bears closer investigation.
Anyway, it’s still indicative of the disrepair of this feature set. It’s still used widely enough that I don’t think it’ll be torn out, but honestly on technical robustness grounds it could warrant being removed.