Apple mail can't open links in Firefox because Classic environment is no longer supported
Manually downloading and installing Firefox and Chrome is a workaround for this problem. It is only when they are installed by brew that there is a problem. I'm not sure what the difference is when install via brew vs installed by the downloaded .dmg
The self updating mechanism mentioned in the comments doesn't work for me. Firefox will tell me that an update is available and offer to download it, but when I click the download button, the dialog goes away and Firefox doesn't get updated.