Important: NoScript add-on is not the answer!
Thanks. Unfortunately, YesScript doesn't provide whitelist functionality at all. NoScript at least has a second-level whitelist, which I still consider insufficient.
There aren't any extensions that allow to enable JS on a URL basis (white-list) apart from NoScript.
If you do that then it isn't possible to enable it in specific cases.
The only way is to enable JS globally and either disable it by default (NoScript) or enable it by default (YesScript) and change the setting for a specific web page.
I was afraid that might be the case. Oh well. I will need to check the possibilities in other browsers. Though it seems pretty hopeless - so far the only browsers than can do what I want are Opera (soon to be dumbed down) and Internet Explorer (not trustworthy at all).
I think I've been using NoScript for over 8 years and can't recall ever seeing it stop working of its own accord. While security concern is healthy, the feared scenario may well be extremely improbable.