Proton VPN (desktop app) blocks Tailscale in Firefox specifically (works fine in Edge, Proton extension has no effect)

Im having a routing issue and hoping someone knows a fix.

Setup is Windows 11 with Tailscale and the Proton VPN desktop app.
The problem is that when the Proton desktop app is connected, Firefox completely drops access to my Tailscale machines. It times out on both MagicDNS and the 100.x IPs.

Edge, curl, ping connect without any issues while the VPN is running. Edge can access the server just fine with the exact same setup.

Here is what I've already tried with zero luck:

• IP split tunneling in Proton (excluded Tailscale's full v4 and v6 ranges)
• App split tunneling (excluded firefox.exe)
• Completely disabling the Proton Firefox extension
• Testing a fresh Firefox profile (so no extensions)
• Testing a vanilla Firefox binary via Playwright and claude
• Turning off netshield in proton
• Verifying Windows Firewall rules (allowing firefox)

It looks like Proton's split tunneling just fails to apply to Firefox specifically. Turning off the desktop app fixes it instantly, but I'd prefer to run both.

Has anyone managed to get Tailscale, the Proton desktop app, and Firefox working together on Windows?