I’ve started using Firefox again as my primary browser. A few versions ago, they included a feature which doesn’t load your tabs until you select them, to make startup quicker.
I find this feature slightly annoying as some of my tabs don’t show if they have new content on them until I click them, which is a bit of a pain (ie I don’t know I have a new email when I launch Firefox until I click my Gmail tab).
I did a bit of searching and it looks like in previous versions, there was an option to enable/disable this in Firefox>Options>General>Show my windows and tabs from last time>Don’t load tabs.
However, that options appears to have been moved, and in Firefox 16, you can find it in:
Firefox>Options>Tabs>Don’t load tabs until selected