The hyperspec keyboard shortcut started launching my email client instead of my browser?

For years I've been using Slime's keyboard shortcut to bring up my 
browser on a local copy of the hyperspec using the standard Ctrl-c d h 
shortcut. Now its started launching a new email window in Thunderbird, 
my email client, instead.

I'm running Ubuntu 18.04, Emacs 25.2.2, and Slime 2.24.

Anyone run into this? Any idea how to fix it?

Thanks.

--Jeff