Well, the fact that it's so random when It happens makes me think it's not really a program using the files, unless ofc, I've been infected by a really smart virus that is using them. I must say I'm an above average PC user and I can identify viruses easily, unless this is like something I've never seen before I don't think i've been infected.
I'll give it a shot to that unlocker tool next time I run into the problem.
*edit* And funnily enough, it just happened again with that same file
Firefox reservs both the unlocker1.9.0.part and unlocker1.9.0.exe files as soon as the download starts, and after the download finished, when it tried to delete the reserved .exe to replace it with the actual file it wasn't able to.