Keep of track of fission

mozilla has recently introduced fission and it is possible to enable it already, but it is not on by default, so let's keep an eye on how it evolves.

current status:

• we have a fission section in the pre-release .cfg I last pushed as a reference, it will be commented out before the next release.
• the are known issues worth looking into.
• fission is currently off in firefox and it will also be in the next release.

future:

• in the best case mozilla enables it in firefox and we don't have to worry about it. at that point the section can be removed from the .cfg as it would be redundant.
• if mozilla does not enable it and we feel like it is ready to be turned on (based on the known issues) the section about fission can stay.