So... Things I've tried so far:
-Reboot : .. mail and safari crash as before...
Re-install MacOS26: ,mail and safari crash as before...
Looking at the stack dump, for both mail and safari, the crash is the same and seems to be related to webkit loading user fonts...
2 WebKit 0x1b216a050 WebKit::addUserInstalledFontURLs(NSString*,...
I wonder if it's some really old font I've had for 10 years that was corrupt or something...