There's obviously a bug somewhere as FF should touch the file every time it starts up/shuts down.
There's a issue on the subject at github where the preference is mentioned.
I'm not sure about it being a bug. A proper Y/N dialog is missing. So you cannot answer N excplicitly, while possibly experiencing Doug's memory issues. If you don't want to give the only button a try, then closing the (poorly designed) dialog is the only option. But closing doesn't have to mean "no, and touch the file". Closing means "I don't want to execute the only option, smashed in my face, now".
A sticking setting may be easier than remembering the settings to restore a different UI language, but I'll consider touching it. Finding the variable path to the file isn't that hard.
I may start liking the ICT way of answering questions, with "something" that "somehow" can be found "somewhere", in a huge collection of "internet stuff"...