The feature actually was there since Firefox 4, but wasn't default. In Firefox 4 it was an about:config setting, sometime later a checkbox was added to Firefox preferences (General, Don't load tabs until selected). I've been using this feature for as long as it has existed, and with the way I use tabs it has been a great help to me.

Pinned tabs load automatically.

I have the feeling that in the beginning, the idea of preserving your tabs was precisely because you wanted them to be prerendered. Then it became a way to have some sort of "front line bookmarking"

I feel like browsers have failed to innovate on bookmarks. The problem with bookmarks is that once you have too many they become unweildy. There should be a separate between "stuff I visit a lot" and "this great article about Early Modern Spain that I want to read later". I'm glad that Safari is delving into the latter a bit now.

Unsorted bookmarks + Awesomebar in Firefox works great for that. It's search-inside-bookmarks is great.

Opera's Speed Dial is nice, too.

I don't know about the Opera version of tabs but in Mozilla 0.9.3 I used tabs because the new window delay was ~2s due to excessive XUL overlays and I installed the save tabs extension because the browser crashed every 2 hours or so and I wanted to keep my tabs.

Exactly, I think they caught on as they are a more intuitive UI than delving into your history or using bookmarks. When rendered repeatedly until you read them (perhaps weeks after,) they make for very expensive bookmarks.

Tabs/history/bookmarks I believe are the most pressing area for UI development in the browser.

A lot of this management could be pushed into the OS shell. If you could manage Windows easier - you wouldn't need tabs.

Tab behaviour is so inconsistent across browsers, apps and different OSs it hurts.