More info on Waterfox.
FireFox VS WaterFox: Is One Preferable Over The Other?
You could try Opera 12.18 Portable. It's pretty lightweight.
Download Opera@USB - MajorGeeks
Note: It's unlikely to receive any updates.
Opera 12.18 is "Classic Opera" with Presto engine (not Chromium/Blink)
It's a security update with a few protocols that are considered unsafe removed and also added protocols which were not in 12.17. and enabled TLS 1.1 and 1.2 by default.
Or try FlashPeak SlimJet (Chrome) / FlashPeak SlimBrowser (IE) - portable versions available.