[achn]

Are there browser extensions that simply disable all future requests from being sent from the immediate tab?

[paulgb]

It's possible for an extension to intercept and block requests, but as Kevin mentions in your sibling comment, it's not enough because they could write data to local storage and then read it later when you're back online if you ever visited that domain again. An extension would have to cover a lot of bases to ensure that data couldn't leak, and I wouldn't trust one to cover them all.

[jannes]

In Chrome dev tools you can set the network throttling to "Offline".

In Firefox this is not possible (per tab), but at least you can set the entire browser to offline mode by clicking "File -> Work Offline"

[jonny_eh]

In Chrome, open the Inspector, go to "Netork", then in the dropdown that says "No throttling" choose "offline".