Why does opening Discord after Firefox crash my Debian laptop?

The order-dependent behavior sounds more like a graphics or hardware-acceleration problem than Firefox simply using too much memory. Both Firefox and Discord use GPU acceleration, so try disabling hardware acceleration in Discord first, then in Firefox if necessary. Also check whether Debian and the Mesa graphics packages are fully updated, and look at the system logs after a crash with journalctl to see whether the kernel or graphics stack reports an error.

Four gigabytes of RAM is tight for a browser plus an Electron app, so check actual memory and swap usage with free -h or a system monitor while launching Discord. If RAM and swap are both exhausted, enabling or increasing swap may prevent a full lockup. Still, a complete crash when the second app opens points more toward the integrated graphics driver or acceleration than ordinary memory pressure.

Try clearing Discord-related site data from Firefox, including its cache, cookies, local storage, session storage, and IndexedDB entries. It may not be the cause, but this has helped with some odd Discord and browser interactions. If that changes nothing, testing Discord with hardware acceleration disabled is probably the more useful next step.