[lollipop25]

First off, use a native app, Gmail or another email client. It's lighter since you aren't loading the UI resources like you would in a web app. It can also be set to store messages locally so that the app won't have to query the server for a message you already loaded sometime in the past. You can always disable live syncing and do manual checking to save bandwidth.

If native apps aren't an option, use Chrome or Opera for Android. They both have bandwidth-saving features. Your pages are crunched by Chrome/Opera servers so that they come in smaller to your mobile device. Should you want to continue using Firefox, Opera offers Opera Max. It's the same data-saving feature on the Opera browser, except packaged as a separate app and does compression on all device traffic where applicable.

Further, I don't think the bandwidth savings are that great if you disable JS. Resources like JS and CSS are cached by the browser. Assuming you don't always clear the cache and let it work the way it should, they only impact is the initial loading of pages where it has to load all resources. Further accesses are retrieved from cache when possible.

Again, use a native app or just use the app the way it's supposed to be used and save yourself the headache. Disabling JS is like trying to drive a car but insisting on having the engine removed.