Same behaviour here too on Linux 6.17.9-76061709. Running Firefox 146.0 on Pop!OS (Cosmic DE) on the main monitor of my workstation. The previous version was definitely not as crashy. I'd love to give more help for troubleshooting because this latest...
This would be lovely. I'd happily get a more "premium" Relay account if that supported being able to send emails from a `@mozilla` provider. I'd probably ditch Proton for it, if a similar set of features could be supported.