master-internet-switch by Elad Karako

start every browser session with internet disabled,
in chromium based browsers, you'll see "website X blocked by web-extension" chrome page.
in firefox based browser, the tab just won't do anything, that's because the blocking in firefox is really "deep", more like a firewall.

anyway,
my main goal was to prevent spike of CPU, RAM (and rendering time),
when restoring previous/crashed session with a lot of windows and tabs,
let the tabs try to load then fail, then zero CPU, very little RAM usage,
and the you (manually) click that button to make the internet great again..
sorry.. I mean allowed.

this is a really tiny web-extension,
based around a 'declarativeNetRequest', and a static "block all" ruleset.
clicking the web-extension's button simply toggle the enabled state of the ruleset (ruleset enabled by default, and initialized, again on session start-up and install, just to make sure the service-worker is awake and button is reactive).

the web-extension has internet disabled (i.e. ruleset enabled),
by default, the idea is to prevent internet from the very early stages of the browser starting-up,
..as much as possible.. see the browser has a little loading scheme,
it loads the ruleset quite early but (I assume) there are browser-initiated requests that may slip anyway, as do requests initiated from tabs, and other web-extensions. so as much as I would like to declare this web-extension is a firewall, it is not (a lot of testing are needed). it does works nicely when restoring a lot of tabs, including UNDO closing of windows and tabs.

I advise turning true those about:config entries:
browser.sessionstore.restore_on_demand = true
browser.sessionstore.restore_pinned_tabs_on_demand = true
browser.sessionstore.resume_from_crash = true
browser.sessionstore.max_tabs_undo = 50
browser.sessionstore.max_windows_undo = 10
browser.sessionstore.enabled = true
(advised)

as for privacy.
the web-extension does not collect any data, not even analytics,
is 100% offline, unaware of the requests, does not modify them,
recent versions (26.9.20+) no host permissions and no content-script.
has no ads. is full, free, and open-source (MIT license):
https://github.com/eladkarako/webextension-master-internet-switch

please feel free to share (in github issues section), test results, feature requests, questions and bug reports.

please enjoy.

donations are unnecessary but appreciated.
https://paypal.me/31adkarak0