Hi Rob, if you check the Downloads list (either click the Downloads button on the toolbar or press Ctrl+J), does the small print under the file name refer to "moz-safe-about"? There have been some threads on those. For example:

https://support.mozilla.org/questions/1375411

No All that is shown is random alphanumeric (Example) hep9kl.htm 0kb

I guess Ctrl+J is needed for this. Is there a site name under the file name on the download list? Usually there is some kind of text there and it could help with further investigation.

I will try the ctrl+j the next time one shows up, and let you know what i get. They seem to be random in nature