Sleep mode for inactive tabs

**Is your feature request related to a problem?**
Some sort of battery saver or sleep mode for in active tabs like Microsoft Edge has currently. To make the browser faster.

**The solution I'd like**
Some sort of battery saver or sleep mode for in active tabs like Microsoft Edge has currently.

**Alternatives I've considered**
None, becease add-ons like [Auto Tab Discard](https://add0n.com/tab-discard.html) discard tabs. This forces the browser to reload the discarded tabs which cost even more performance resources.

**Additional context**
https://github.com/black7375/Firefox-UI-Fix/issues/2#issue-870942029
> * [ ] Battery saver

The issue describes a browser-level sleep or battery-saver mode for inactive tabs and references the Firefox-UI-Fix issue discussion. Start by determining whether this repository can implement the requested behavior; the issue names no files or tests, and completion criteria are not defined.

Written by the indexing model from the issue text.