On 22 Mar 2020, at 16:01, Jeffrey Cunningham <jeffrey@jkcunningham.com> wrote:

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?

You need to identify what emacs package you loaded that overrides the C-c d h keybinding.
Note that slime-documentation-lookup is normally bound to C-c C-d h 
So C-c d h must have been put there by yourself in your ~/.emacs 
Try to do that at the end of your ~/.emacs, to get a chanc to override the key binding yourself.

-- 
__Pascal J. Bourguignon__