That did the trick!

Reinstalled Firefox keeping my old settings.   Now the hyperlinks in PMail open in FF again.

Thanks

After the recent Windows 10 Fall Creators update, I found that my hyperlinks in Pegasus Mail (v 4.72.572) no longer open my default browser.   Instead it opens Windows Explorer at the windows/system32 menu.   I have tried changing the various settings in Pmail for hyperlinks.. including pointing directly the my browser exe file (FireFox).  I have checked in Windows settings for defaults for browser and http and https.  All point to Firefox.

Help! 

First thing to try is a machine reboot, full shut down, not just a restart.

If that doesn't work, change your system default browser to IE, do a machine restart, then change it back to Firefox.  We think this toggle resets registry entries that Windows updates mess with.

You might also want to look at whether the .htm and .html extensions remain set to Firefox.

I've done full shut down a few times over the last few days.

Changing my system default browser to IE, the hyperlinks will open in IE. But the .htm and .html extensions remain set to Firefox.   Changing the default system browser back to Firefox and it goes back to opening the windows/system32 directory in Windows Explorer.

If I click on a .htm or .html file in windows explorer it open in Firefox as it should.

The other thing I have noticed, if I right click the hyperlink an select "open in browser" it only gives me IE and Chrome as an option.

[quote user="BethM"]The other thing I have noticed, if I right click the hyperlink an select "open in browser" it only gives me IE and Chrome as an option. [/quote]

That list is pulled from the system so I suggest an uninstall/reinstall of Firefox.