That sounds like a really useful option, but I don't see anything along those lines when I look around for it.

The DevTools team has its own forum where you could check on that and/or tell them this is important to you:

https://discourse.mozilla.org/c/devtools/213

As a crude workaround, you could disable all extensions from injecting scripts into https://localhost/ pages by adding localhost to the restricted domains list. Please be careful to not remove Mozilla domains from this list because they are there to protect against add-ons gaining elevated access to internal features or Firefox Account data.

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

More info on about:config: Configuration Editor for Firefox. The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.

(2) In the search box in the page, type or paste extensions.webextensions.restrictedDomains and pause while the list is filtered

(3) Double-click the preference to display an editing field, click at the end and add a comma, then the host name of the site you want to shield from extension interference (I add financial institutions here, for example).

Down the road, when Firefox supports "Manifest v3" add-ons, you'll be able to opt sites in and out of running individual extensions. Can't come soon enough.

Thank you greatly for your time and such detailed answer!

I sure will leave a feedback on dev tools forum for that. I see that there's a similar question there regarding CSP errors thrown by the extensions (to be frank, that was the main issue I was facing, I just was not diving into details to see what kind of resources extensions request).

Turning off specific add-ons for specific websites would be the best solution though for sure.

You could also create a separate profile for development, without addons.