I'm having exactly the same problem and it's very irritating! Same version of Firefox and I also have changed nothing. You're not alone - anyone know anything about this?

I have the same problem, but I'm using the right side chat. For some reason, the inbox is not automatically resizing itself the way it used to.

same problem here with the right side chat. You can click the "hide" bar of the RTM extension to show the right side chat or labels, but this is not really ideal. Up until last week, the RTM extension caused the the right side chat/labels to move left and display next to the extension, rather than under it.

Unfortunately some recent Gmail code changes mean that RTM for Gmail isn’t currently compatible with some Gmail Labs features (you might see the tasks pane overlapping incorrectly with the Gmail message pane).

We’re investigating support for these Gmail Labs features; in the meantime, please try disabling Labs if you encounter problems.

I'll post any additional updates that we have in this status blog post.