Why does opening Discord after Firefox crash my Debian laptop?

Please compare the versions of Firefox, Discord, Mesa, and the kernel, then install all available updates. Older packages can contain graphics or Electron bugs that have already been fixed upstream. If the problem continues, launch Discord from a terminal and inspect the output, and check the previous boot's logs after restarting; those details should distinguish an out-of-memory event from an Intel graphics or application crash.

You could also clear Discord-related site data from Firefox, including its cache, cookies, local storage, session storage, and IndexedDB entries. That is more likely to fix a browser integration or corrupted-data issue than a full system crash, but it is a quick thing to test. Make sure you know any passwords that may need to be entered again afterward.

Four gigabytes of RAM is fairly tight for a modern desktop, even with only a few tabs. Firefox can sometimes use several gigabytes by itself, while Discord commonly adds hundreds of megabytes. Watch memory and swap usage in System Monitor while launching Discord. If swap is missing or very small, adding swap could help with memory pressure, though a complete system crash may point to a driver problem rather than RAM alone.

The GPU and graphics stack are worth checking, especially since both Firefox and Discord can use hardware acceleration. Try disabling hardware acceleration in Firefox and Discord, then test whether the crash still happens. Also check system logs after rebooting so you can see whether the kernel, graphics driver, or out-of-memory handler reported anything.