Inactive Firefox uses too much CPU (10% .. 30%)

Question

On Ubuntu 14.04, I have a Firefox 46 with 30 tabs open. The Firefox window is minimized, I've been working with other programs in the last 3 hours. In top I can see that Firefox still uses between 10% and 30% CPU.

I think this idle CPU usage is excessive and unreasonable. I need the CPU and the battery power for other programs. I don't want to exit Firefox for convenience: because I may need any of the tabs any time soon. I'd like to suspend CPU activity (e.g. for running JavaScript, running flash, playing videos, animating images) on tabs which haven't been active in the last 5 minutes.

Is there an extension for that?

• Suspend Tab doesn't work, because it just flushes the caches, it doesn't suspend JavaScript.

• UnloadTab doesn't work, because it makes Firefox forget the entire tab state, i.e. any unsubmitted form input will be lost.

• Auto Unload Tab doesn't work for the the same reason: because it makes Firefox forget the entire tab state, i.e. any unsubmitted form input will be lost.

• Suspend Background Tabs doesn't work, because it was designed for an earlier version of Firefox, and it has been removed from the addons website, so it's unavailable.

For debugging purposes: how can I figure out the average CPU usage per tab? Is there an extension for that?