tl;dr: Support on-demand activation (and deactivation) of user-selected extensions through single click; it could improve memory consumption and page loading speed; many types of extensions will still work correctly.
At the moment, extensions are either enabled or disabled. If the user has lots of extensions that may be used, he/she would have to enable them all the time, or spend much time looking for the disabled ones in the long list.
In fact, many extensions do not rely on analyzing nor changing the browsing page's content. For example:
Search/Add the current page from/to somewhere (e.g. webarchive)
And some extensions work only relying on a minimal set of information that can be obtained from memory upon request. For example:
Display URL information (including showing QR code)
Show webpage's heading map (requiring the page's source, which is available in memory)
Add a "search field" of the page to search engine list
In principle, these types of extensions can be activated on-demand when the user needs, and will function normally. This has benefits on resource consumption as well.
I would suggest allowing users to specify extensions they would like to load on-demand. The UI to toggle the activation of on-demand extensions can be something similar to the current "overflow menu".
Having this on-demand loading of them could save memory usage when they are not activated, and speed up page loading time.
Regarding usability and/or breaking extension, the browser can give suggestions if the extensions will likely function abnormally. This should be doable based on the permission requested by the extension. Of course, this analysis won't be complete, so the user should be the eventual reliable source for this.
This has some similarity to https://connect.mozilla.org/t5/ideas/option-to-allow-extensions-to-run-only-on-specific-websites/idi-p/146, which is requesting to only activate extensions based on website. This can be understood as setting the "on-demand" state based on website.
This also has some sort of link to https://connect.mozilla.org/t5/ideas/enable-disable-extension-quickly-via-firefox-bookmarks-menu/idi-p/5309, which is another proposal to quickly toggle the activation state of some extensions. Though the focus is different.
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