Bugzilla

> As I wrote, I've filed a new but report just because of bad experience with the old(er) ones being neglected.

In the world of bugzilla.mozilla.org, we file new bugs for such reasons and newer bugs covering same territory get duped back to the older bug - unless (sometimes) the older bug is a tangled unmanageable mess.  Such is not the case with bug 683651 nor bug 686495. 

> should easily reproducible as mentioned above

Should is a horrible word. I don't doubt the behavior you are seeing, but just because you see it, and can even trivially reproduce it, doesn't mean it is simple to reproduce or that someone else "should" always be able to reproduce it.

> this must be a rather low-level issue

"must" is another bad word. We have no hard facts on which to base such a conclusion.

My laptop is i7 quad core. My brief CPU increase of 1-4% is only one core (and a completely acceptable amount of increase IMO).  I'll comment in the other bugs.

> As I wrote, I've filed a new but report just because of bad experience with the old(er) ones being neglected.

In the world of bugzilla.mozilla.org, we DON'T file new bugs for such reasons, and newer bugs covering same territory get duped back to the older bug - unless (sometimes) the older bug is a tangled unmanageable mess.  Such is not the case with bug 683651 nor bug 686495. 

> should easily reproducible as mentioned above

Should is a horrible word. I don't doubt the behavior you are seeing, but just because you see it, and can even trivially reproduce it, doesn't mean it is simple to reproduce or that someone else "should" always be able to reproduce it.

> this must be a rather low-level issue

"must" is another bad word. We have no hard facts on which to base such a conclusion.

My laptop is i7 quad core. My brief CPU increase of 1-4% is only one core (and a completely acceptable amount of increase IMO).  I'll comment in the other bugs.