That could simply be the cookies.txt file with no content whatsoever.
Firefox
A place to discuss the news and latest developments on the open-source browser Firefox
Sounds like it's localStorage. But I'd expect that to be covered by "site data" in that option.
It's a bit like cookies, but just for one site. Some think they can avoid cookie consent banners with localStorage.
Firefox has a page on the topic.
What is the data it's keeping? 25-100 bytes doesn't seem like there's anything actually there.
8 bytes is enough to store 18,446,744,073,709,551,616 (18 quintillion) different values, more than enough space for a fingerprint ID. The size of the data shouldn't factor into the potential threat.
Oh for sure, I just wonder if the size being the same on 2 of them is a result of Firefox storing some default data there on cleanup by accident.
Oh, I get it now! In my sleep deprived state I missed that two of them had the same size. That seems like a reasonable guess, I'm just paranoid about cookies :P
Except the actual content is very likely boilerplate stuff.
My point still stands. The size of data says nothing about its contents. If OP is concerned about this from a security or privacy perspective, you shouldn't be writing them off because it's only 100 bytes.