I don't think this is right. Firefox got a major foothold because it blocked ads.
This was right as those ads that would open 4 new windows when you closed a previous window started showing up on the web and IE6 could not handle them. It would literally kill your browsing experience and sometimes blue screen your computer. Firefox was the first mainstream browser to block pop ups out of the box and it worked.
It was annoying, but that is what I was used to. Then Firefox came and allowed to have all your browsing in the same window, without polluting the taskbar.
In my case, I clearly remember that tabs were the reason to switch into Firefox.
But boy, IE6 was painful! And i needed tabs to open pages and do something else while I wait for pages to load. That made me switch to Firefox after Opera. It wasn’t that long ago, I think 2007 sorts?
In my job now, I still try to make sure people can access my pages with as little resources as needed.
This is how I remember it: Firefox didn't block popups on its own, but they were constrained to another tab instead of a new window, which alone was a huge bonus.