This issue annoys me, so I searched for a workaround which I found :) In Icecat go to about:preferences -> General -> Applications (only a header): Set mailto to `Use other` and set it to `/run/current-system/profile/bin/icedove`.
$ grep mailto /gnu/store/3zxx08qfl9q6d9s80p58521y9x3ni3p1-icedove-78.3.3/share/applications/icedove.desktop MimeType=x-scheme-handler/mailto; That seems correct. xdg-open mailto:f...@gnu.org works for me always, so it really seems a Icecat/Firefox thing...