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