in Thunderbird, running KDE, when I click on a link in an email message Quantas instead of Mozilla starts up.

I don't know how this happened. How can I change it back?

Probably - change default kde browser

would you happen to know how to do that?

oops, its Quanta. Doesn't show up in the the software removal area.

I trhed this - it has zero effect.
KDE
Open kcontrol (KDE configuration center).
Go to "Components -> Components chooser -> Web browser".
Check "The following browser", and type in "firefox" ("mozilla" for Mozilla).
If you don't notice the "Web browser" component:

Go to "Components -> File association -> text -> html".
Select "Add..." under "Application Preference Order".
Write the command firefox %U (mozilla %U for Mozilla) and select OK. You need "%U" so you can load URLs that are non-local files

Open KCC (The KDE Control Center). Navigate to 'KDE Components' / 'File Associations'. Open the 'text' tree and select 'html'. Highlight the browser you want to open when you click a URL, and click the 'Move Up' button at the right until your chosen browser is at the top of the list. Click the 'Apply' button near the bottom right then close the window. Now when you click a WEB link, the browser of your choice should open it.

HTH,

Done.
No effect.

It is some strange. And if you type in console
$ $BROWSER
Which of the browsers will start?

Strange... it must be something with Thunderbird.

That works fine - it opens in Mozilla.