Update: Thanks to Eric McIntyre for writing to point out that there's a built-in workaround that does something like the above. Firefox can permanently save multiple home pages. If you choose this option (at Tools > Options > General), every time you launch the browser it will open all the pages you've configured as your home page. This isn't precisely what I'm looking for. I want the browser to be able to save the last however-many pages I was looking at and reopen them the next time I run the program. It's useful for when you have to shut down suddenly but want to pick up where you left off. Nevertheless, the multiple home pages thing may work for some people, so it's worth mentioning.

Like a lot of things in Firefox (and Mozilla 1.7.3, which works similarly on this score), this is a head-slappingly obvious solution, seemingly elegant in its simplicity. But this one just doesn't work for me. I don't want to clutter up my already voluminous bookmarks with a whole bunch of tab sets. Also, you need to go to extra trouble to store tab sets neatly in bookmarks. There's no special Tab Set special folder that tab sets are automatically stored in — that might help a little. Of the many features that Mozilla opted to streamline for Firefox, the company would do well to go back and rethink named, saved tab sets for a future version.

Happily, a Firefox Extension called Session Saver was designed to provide exactly this functionality. Session Saver hasn't been updated since before Firefox 1.0 was released, but a specific version of Session Saver, offered on the Extension Mirror website, installed and worked properly for me with Firefox 1.0. (I had minor difficulty with the other versions I've found.) Session Saver works to both save tabs between sessions and also provides a way to name and save tab sets. It's a highly useful Firefox extension.

But for those of us who prefer to greatly limit the number of instances of the browser program, it doesn't provide a default behavior for one of the more prevalent causes of new-browser opens. Webmasters can easily force a new instance of a browser by adding a simple attribute to any hyperlink on any Web page. What Firefox still needs is an additional option to "Open tabs instead of new windows for links on a Web page."

Thankfully, there are some easily-configured customizations that neatly solve the problem. The Mozilla Firefox Support site offers instructions on how to Reveal More Tab/Window Options. This modification adds a new Tabbed Browsing setting in the Tools > Options > Advanced dialog that controls Web site-forced new browser opens. After you make this tweak, close and restart Firefox, and then open Options > Advanced and put a check in the box beside "Force links that open new windows to open" and make your preferred behavior choice. A second Mozilla customization, Decide Which New Windows to Block, allows small pop-up windows spawned by JavaScript to open as normal pop-ups instead of as new tabs in Firefox (which sometimes forces your entire browser window to shrink). I strongly recommend adding this customization too. Being able to find quick solutions to petty annoyances is one of my favorite things about Firefox.

There's also a Firefox extension called Tab Clicking Options by Twanno that gives you additional control of your tabs, including being able to double-click any tab to close it. It let's you configure keyboard-mouse click combinations, such as Ctrl-click, to open, close, close all, duplicate tabs, and other functions. I definitely recommend it.