Removing the file and $HOME works perfectly.file:///home/<username>/'https://www.student-finance.service.gov.uk/<redacted>'
I examined the running system and the command line (from ps -ef | grep fire was:
From a terminal both the command firefox 'https... and /usr/lib64/firefox/firefox 'https... both worked perfectly. Going into /proc shows that the commandline is just as ps displayed./usr/lib64/firefox/firefox 'https://www.student-finance.service.gov.uk/<redacted>'
I'm a little stumped. It appears the Thunderbird is sending the right string, and yet Firefox consistently applies a default "file://$HOME" to the start of the string when invoked this way.
Any pointer please!