Google has released a new version of Gmail. The ui parameter that appeared in the URL recently, the one that broke the HTML Reply Signatures for Gmail Signatures Firefox Extension and Greasemonkey, was part of the plan for allowing users to switch between the newer version and the older version.
How to Go Back to Gmail's Older Version explains more of the details regarding the new version features as well as how to switch back to the older version.
I just bought a 1968 Ford Thunderbird! I bought it yesterday actually. It hasn't even been 24 hours! It has a 429 cubic inch big block V8, suicide doors, a nice, well-preserved leather interior, and driving this well-engineered machine is like stepping back in time! It also has enough power to propel you into tomorrow!
And just how does this relate to the Gmail Reply Signatures Extension you ask? Well, it will be awhile before I will be able to fix this newly created Gmail bug, so for now, I'm going to recommend that you use the older version of Gmail so you can still use your HTML Signatures. Fortunately, Google is good at allowing us to enjoy backwards compatibility for awhile.
I'll post pictures of the car soon!