(In reply to Anje from comment #8)
> ...
> I'm not sure whether setting:
> THUNDERBIRD - Change the Preferences
> Files & Attachments
> Firefox HTML Document from Firefox to Adobe Acrobat
> testing opening of pdf
> Then resetting it back to say Action: Firefox default
> Seems to be fixing the issue
> ...
(In reply to Anje from comment #8)
> ...
> I'm not sure whether setting:
> THUNDERBIRD - Change the Preferences
> Files & Attachments
> Firefox HTML Document from Firefox to Adobe Acrobat
> testing opening of pdf
> Then resetting it back to say Action: Firefox default
> Seems to be fixing the issue
> ...
Thank you, Anje!
Your way worked for me! Hooray!