(This is dictation.)
Here's today's problem. Actually, I guess I've had this problem for quite a while, not sure about that. Anyway, Internet Explorer now wants to be the default browser that I have to use when I open my e-MAIL. Let me explain what I mean.
I like to use Thunderbird portable as my default e-mail client. I'm familiar with it and have come to learn about some of its quirks and behaviors. I get messages from other people that know me and they asked me to join them on Facebook or LinkedIn and when I try to click on the link it gets messed up because it strives to start Internet Explorer. For me Internet Explorer is a complete DISASTER. I have no idea what it's doing, I am accustomed to using Mozilla Firefox. This is my computer, I paid for it, and I bought the software for it and never conceded to Microsoft that it has the right to take over the way I use my computer. When Microsoft takes over my computer, I'm not able to use it.
Now here's the problem. I cannot find the area where you tell Thunderbird what is the browser is supposed to use the following link. I've been told that sin an area CALLED preferences. But I can't even find where the preferences are in Thunderbird. I just don't see them. Where are they?
Thank you very much for your attention.
Microphone off
(End of dictation.)Thunderbird uses the default browser of the system.Thanks, BC
I tried that twice and still when I click on a Facebook notification it starts up in Internet Explorer and not Firefox.
To work around that, I have to "copy link" and drop into Firefox. Then it takes me to the Facebook notification.
I will just have to live with it. A few more mouse clicks. 

Maybe this is a quirky thing with XP.
When I do e-mail,  I prefer working inside of XP. Otherwise using Windows 10  creates too much stress and runs my blood pressure up. 
He means you don't have FireFox set properly as the default browser in XP...double check that setting...And Yet More Info...

Your incoming mail or attachments are chamging the default settings...Thanks PATIO, that article looks to be heavy lifting...  It is...my apologies...

But it's do able...OK. I am trying.
But for sanity check I looked at something else that says I  should try my administrator account. So I do. guess what? In the administrator account there is no problem when I make Firefox the default. Then thunderbird works well.

Why would that be?   

BTW: I am using Thunderbird portable, so  I get the same e-mail settings no matter what user I login. (A behaviour found in portable applications.)

EDIT: Update. crated new user account. FireFox  is default. Thunderbirtd works OK.
Sooo, me thinks itwould be easiertotreat this as a corrput XP user account. Which I have delt with before. I will try that.  Problem solved. Somehow something reseating the default browser right after I would set it to Firefox.
Very odd. This is a recent install of XP pro with update stuff.
I had to fix the corrupt user account. No clue as to what went awry.

Thanks for the suggestions and the leads. 
GOOD News...glad your fixed up...