I can’t save the image to my disk due to “illegal characters” in the name of the saved file. I think it is due to the characters in tab’s title.
It happened on two sites which I tested:
On the first site, the illegal character was probably question mark ("?") or slash ("/"), and on the second one probably colon (":")
Screenshots from other sites (eg. https://www.reddit.com/r/firefox/) save to disk without problem.
I think Firefox should remove such characters or substitute them with some other character – instead of throwing an error. This is a pretty serious problem.