Firefox times out at 30 seconds or gives a message that it can not fetch a resource.

[Koepel]

It was mentioned in the Discord channel that the Firefox browser stops the build process after 30 seconds.
At that time it was not consistent for me, but now it is. Every ESP32 (or variant) with Arduino times out at 30 seconds, but sometimes at 60 seconds.

Today, sometimes this error shows after 30 seconds:

The build process is normal when using the Chrome browser. It tries to build up to 100 seconds.

[urish]

Thanks for reporting - I reproduced with the following project.