Hi!

Does the message get stuck? Are you able to keep writing?

Sometimes this message appears to early but once you write the whole e-mail address it disappears and you can continue normally.

Take a look to the screenshoots attached to understand what I mean.

Thanks Ibai.

Unfortunately this did not help. After the first letter of the email address I still get the "Invalid email address" advisory as well as an "Unknown error" message. These persist even if I continue typing the full email address and fill in both password fields. The "next" button stays greyed out.

The fact that this is happening on two different computers with two different operating systems is very strange since it implies that this is a bug within Firefox itself.

I also find it odd that no one else has noted this problem?

That's not normal.

Is it a fresh Firefox 5 install? In other words, do you have any add-on? Do you have the Sync add-on installed? We are seeing some conflicts when people has the Sync add-on in top of Firefox 4 and 5 (they already include Sync and there's no need of an add-on).

This is the first time I read about this behavior, understanding the configuration would be great.

Give a try to the Safe Mode and let see if it helps.

OK, just did a complete uninstall of Firefox followed by a reboot and a reinstall from the download at Mozilla.com. I have no add-ons at all. The problem is still there exactly as it was before. I tried safe mode too and that didn't help either.

Wierd indeed...

My only recommendation then is to open a bug in Bugzilla under Firefox Sync UI so the developers can take a look to it.

It would be awesome if you can share with us the outcome.