Add Firefox Flatpak support

[noverby]

Fixes #409

[compiaffe]

Very nice! @filippobuletto can we get this merged, please?

[filippobuletto]

Surely you mean @FiloSottile :)

[compiaffe]

Haha, yes I absolutely mean @FiloSottile :)

[compiaffe]

@FiloSottile friendly ping :)

[francoism90]

I think the project is dead. :/

[bugnano]

@FiloSottile friendly ping :)

[lossendae]

@FiloSottile friendly ping :)

[noverby]

The project seems to be dead, so I'm closing this.

[bugnano]

@noverby I wouldn't have closed this issue.

Yes, this project looks abandoned, and I also asked if it was on #548 .
According to @gaby the maintainer of this project, every now and then remembers that this project exists.

By closing this issue, you only prevent this PR from ever getting merged.

What was the problem of keeping this PR open?

[rfay]

Yes, it seems like he stops by about every 2 years :)

[noverby]

@bugnano I thought that there was no chance of this being merged,
but you convinced me otherwise, so I will open it again.