Bugzilla

Windows 10
Thunderbird version : 91.5.0

Testing:
Changed Preferences for pdf to 'Always Ask'

Select to open pdf attachment - a window opens asking what to do.
Select 'Open With' and click on drop down which offers  'Adobe Acrobat (default)' and 'Other'.
I click on 'Other' - see larger list which includes Thunderbird.

Select 'Thunderbird' then it opens a new 'Write' window and the pdf is shown as an attachment.
Obviously this is not desired. 

Select to open again - I now see 'Adobe Acrobat', 'Thunderbird' and 'Other'
'Other' offers quite a list including 'Firefox'
This does open the pdf in a new Firefox tab.
OK, I can read the pdf.

Select to open again - I now see 'Adobe Acrobat', 'Firefox' and 'Other'
So at least the list gets populated with the default - adobe and also the last selected option.
That's good.

Result:
You would expect the 'Thunderbird' option to do the same as the 'Firefox' option - open pdf to read in a new tab in Thunderbird, but it does not and it does not offer 'Preview in Thunderbird'.

Windows 10
Thunderbird version : 91.5.0

Testing:
Changed Preferences for pdf to 'Always Ask'

Select to open pdf attachment - a window opens asking what to do.
Select 'Open With' and click on drop down which offers  'Adobe Acrobat (default)' and 'Other'.
I click on 'Other' - see larger list which includes Thunderbird.

Select 'Thunderbird' then it opens a new 'Write' window and the pdf is shown as an attachment.
Obviously this is not desired. 

Select to open again - I now see 'Adobe Acrobat', 'Thunderbird' and 'Other'
'Other' offers quite a list including 'Firefox'
This does open the pdf in a new Firefox tab.
OK, I can read the pdf.

Select to open again - I now see 'Adobe Acrobat', 'Firefox' and 'Other'
So at least the list gets populated with the default - adobe and also the last selected option.
That's good.

Result:
You would expect the 'Thunderbird' option to do the same as the 'Firefox' option - open pdf to read in a new tab in Thunderbird, but it does not and it does not offer 'Preview in Thunderbird'.

'Preview in Thunderbird' option can only be set up for pdf in the Preferences > General
But if someone needs to sometimes open in Adobe or other pdf software they would select 'Always Ask' and this would be ok, but would discover it impossible to select 'Preview in Thunderbird' for other occasions.