For Firefox there is WebDeveloper extension https://addons.mozilla.org/ru/firefox/addon/web-developer/ which has menu to disable browser caching. In IE8 / IE9 there is menu to disable caching when you run debugger via F12.
That's true for me, while developing.... but if I change the WikiEditor toolbar, my 3000 users won't see the change.
I must agree with you that modern browsers are caching the content beyond the evil...
Maybe WikiEditor is doing something special (or not doing something) for the caching to be this aggressive? Thousands of other web sites will properly update if you do a ctrl-F5 (hard refresh). Many MediaWiki extensions do too. But WikiEditor does not: you have to manually erase the browser cache, not only in Firefox but also in IE and Chrome.
DanB