Upstream is building the binary 'thunderbird' but install this as
'thunderbird-bin' and is doing later some magic to also copy
'thunderbird-bin' back into 'thunderbid' inside the installation folder.
It looks more naturally Thunderbird needs to be started by calling
thunderbird-bin in the wrapper script, particularly as the binary is
doing some self checking how it was called.
Some users reported they had issues to start Thunderbird by calling

  /usr/lib/thunderbird/thunderbird

but did have success by calling instead

  /usr/lib/thunderbird/thunderbird-bin

The starting wrapper script got modified in that way to always call now
'thunderbird-bin'.

Closes: #882122