[devtools] qunit: only start firefox once per test
The separate profile initialization step doesn't seem necessary anymore.
This was due to the system-wide installation of the "Sage" Firefox
extensions which breaks -url on a fresh profile. Removing this obnoxious
extension allows us to launch Firefox in a standard way.
Related to #5533333.
"""cubicweb-%(cubename)s application package
%(longdesc)s
"""