One possible approach to the free space issue on a laptop could be (encrypted) swap. If the system is configured to allocate swap pages in order and you have a

One possible approach to the free space issue on a laptop could be (encrypted) swap. If the system is configured to allocate swap pages in order and you have appropriate limits in place, the likelihood that you reach 100% swap usage can approach zero (but is hard to prove). Use the free space at end of your swap for secrets. Having multiple swap files / swap partitions that you activate when you need more memory for compiling Firefox would give further proof against accidentally overwriting your secrets.