perhaps https://support.mozilla.org/en-US/kb/installing-addon-thunderbird

'[Ignore this response - I can't find a way of Deleting this. See my response further below].'

Thanks Matt for your reply - I'll try and give this a go and see how far I get!

Not being a skilled computer techie sort of person, things like downloading/installing always scares me a bit, as does fiddling with things like config editor, and HTML and stuff.

Thanks Matt for your reply - I'll try and give this a go and see how far I get!

Not being a skilled computer techie sort of person, things like downloading/installing always scares me a bit, as does fiddling with things like config editor, and HTML and stuff. And, I'm afraid I find it almost impossible to find specific answers to my specific questions in the maze of info that most links have taken me to, and just end up confused!

LATER ADDITION Matt, I'm sorry, but I couldn’t find the answer to my particular question in the link you gave me.

All I was wanting to know was what to do when a message asked: : 'How do you want to open this file?' Well - I haven't a clue how I want to open this .xpi file. Nor have I a clue what app I should be looking for, in order to open .xpi files.

And might it be affected by what versions both the download (v1.10) and my version of Thunderbird (v 52.3.0 (32 bit)) are?

What were you doing when getting the 'How do you want to open this file?' message?

Edit: Looks like there already was some information in your OP.

I then clicked an option to 'Open', and got asked: 'How do you want to open this file?'

In any case, please be more specific about where and what you clicked.

Lets back up to the very beginning here. Why do you think you need this add-on? Given your stated abilities I really doubt the add-on will solve whatever perceived problem you think it will.

Second. Use the instructions I gave you to locate the add-on using Thunderbird's add-on manager (the ideal case) or use Thunderbird add-on manager to open the XPI file (the less ideal case)

In either case you can not get windows to anything useful with an XPI file in a meaningful way by double clicking on it. The windows system is just to stupid. Making it do something with those files for Thunderbird will break the process for doing something useful for Firefox. So you only have the two methods described in the article I linked you to.

Many thanks Matt and Christ1 for all your detailed replies (which, along with any links you've given me) Ive copied and pasted into my Docs for reference), and I'm sorry to have taken up both your times like this.

Thing is, I've completely lost track of why and how I started all this .xpi stuff, so I think, for all our sakes, we'll wrap this one up and consider it solved!

Many thanks again.