Yes, I do! Firebug is so innocuous, it never occurred to me it might be causing problems. I'll remember that the next time I'm downloading a big file.

As for download managers, it seems my opinion of that class of application -- which was formed back when they were little more than vehicles for banner ads -- is now obsolete, and I should give them another look.

I personally would blame chance rather than firebug. It just doesn't add up that an add on could exhibit the behaviour.

I'm sorry that you have a problem, but I wouldn't suggest kicking whatever might be there. I have firebug installed, and am fine. A big download will sometimes be corrupted, this is more likely the fault of yout internet than the browser. Firefox using memory is not an indication of bad use of memory, as you have pointed out, it releases that memory later, thus it has used it and discarded it.

I would suggest checking your connection, and testing other downloads. It's just my guess, but I don't think it's GOG.