Greetings, albrumbach.
Since your Mail links are opening in Safari, it’s best to start by checking to be sure that Firefox is still set as your primary browser:
“How to change your default web browserMake sure that the other web browser is installed.Choose Apple menu > System Preferences, then click General.Choose a web browser from the ”Default web browser” menu.How to change your default email appMake sure that the other email app is installed. Although you might be able to use a web browser for email (webmail), a web browser isn't an email app.Open the Mail app.Choose Mail > Preferences, then click General.Choose an email app from the ”Default email reader” menu.You might be prompted to add an email account before you can change the setting in Mail. If you don't want to do that, check the preferences of the other email app. You might be able to set a default email app from there.” Change the default web browser or email app on your Mac
If so and the issue continues, try switching your browser to Safari and then back to Firefox and test to see if your links open in the correct browser.
We hope this helps!
Sincerely.