At some random point today, I started going to various web sites and they seemed to be falling back to basic views without anything dynamic going on. For example, in GMail, I saw "Some important features may not work in this version of your browser, so you have been redirected to the Basic HTML version. Upgrade to a modern browser, such as Google Chrome."

Weird, because it was just working a second ago.

Realizing it was not properly detecting my browser, at first I thought GMail just didn't support Firefox 8, then I thought, "No, that's silly, something else must be up. Let me check my user agent string."

So, really quick I hit whatsmyuseragent.com and my user agent string was showing as

undefined GoogleToolbarBB

WTF?!

A quick venture into about:config in Firefox and I saw that the general.useragent.override preference was set to exactly that - undefined GoogleToolbarBB.

I reset that to default (empty) and now everything works again.

I have no idea when (or why) that happened, but if you see the same thing, that's how you fix it.

Note: Seems I'm not the first person to see this.