I am working with Ubuntu 18.04 and notice a strange behavior of programs like firefox, thunderbird or chromium:
If I open firefox, it typically takes maybe two minutes until it starts. Same with thunderbird and chromium.
Now here is the strange thing: I start firefox - nothing happens now I start chromium - immediately, firefox opens, chromium does not now I start thunderbird - chromium opens.
It works in all combinations. The first program waits until it is triggered by the invocation of another program.