My theory is using tabs would use less resources then having the same sites opened in separate windows. My reasoning is each window has to load the entire browser interface, not just the page content. So, I did a little experiment. First using Firefox 18.104.22.168 with a clean profile (no extensions, no plug-ins and default theme) and imported my bookmarks from another profile.
For the first test, I opened Firefox with the clean profile and choose 5 sites and opened each one in its own window:
Next, I closed then opened Firefox with the clean profile and choose the same 5 sites and this time, opened each one in its own tab:
As you can see from the two experiments, having the same sites loaded in tabs used 25 MB less memory. Okay, so 30 MB may not be all that much, but remember this was with a clean profile. There are no extensions load and I was using the default theme.
Okay, so how about something a little more typical? This time I have used my default profile which still uses the default Firefox theme but with 32 extensions. I tested with 7 tabs open (6 with pages loaded and 1 blank). Then tested the same 7 pages each in there own windows. Here are the results using a normal (with extensions and plug-ins) profile with 7 pages (1 blank) in 7 Windows:
The same profile with the 7 pages in 7 Tabs:
Even with a more typical usage (profile with extensions and plug-ins) Firefox is still using more memory (30 MB) displaying the pages in their own window instead of in tabs on one window. But even an extra 30 MB is not all that bad for memory usage. It really comes down to personal preference, but I the big “selling feature” of Firefox was the tabbed browsing.
Edit: While totally unintentional, it was interesting to see that on 3 out the 4 test Flock which was sitting idle with 4 open tabs was using more resources than was Firefox. This version of Flock is based of Firefox 1.5, which as many will recall was heavily plagued by memory leaks.