I actually use 32-bit Firefox for this reason, it puts a limit of around 2GB per tab. I would like to switch to 64-bit though if they added this feature in about:config, because the main Firefox process in about:processes also gradually leaks memory ...
Please just give us an "browser.tabs.warnOnClosePrivate" about:config option or something. I use Private Windows a lot for temporary cache and cookies. Tempted to just finally get around switching over to Vivaldi for this.
