Running Firefox in background causing guaranteed client crash

[Enkidoh]

Thirded, I use Firefox also and although I don't keep it running in the background all the time, the times I do use it when running FFXIV I've had no issues like this.

I do know the latest version of Firefox can be a resources hog though especially if you're running lots of plugins and apps with it, so if your laptop only has a small amount of memory, Firefox may be clogging up the RAM, and more often than not will crash as a result, especially if you're running it at the same time as another program like FFXI (even more so with FFXIV given that games are resource intensive by their nature).

But without knowing your system specs it's hard to say (laptops are rather more prone to this due to tending to have more limited RAM and disk space).