Thank you very much for your replies, Sonia. I'm using Fedora, rather than Ubuntu.
Following Applications>Preferences>More Preferences, I get to Preferred Applications. When I click on it, I get a window with a number of tabs, one of which is called Web Browser.
On that tab, I'm able to select my default web browser. I've got the Custom Web Browser button highlighted and a box into which I'm to put a command. The command which appears is the full path to the Firefox executable. I've tested that command from the command line and it does open Firefox. Unlike what I understand to be your situation, however, there is no option available on that tab relating to how links are to be opened.
It's occurred to me since posting my original query that maybe there's something that's changed without my knowing it in the Firefox configuration editor and that that's what's caused me to lose the automatic opening of links.
In the absence of any other suggestions, maybe I should have a look at that (though there are an awful lot of entries in it, aren't there!).
Thanks again, Leslie -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html