Any particular sites?

Four things come to mind:

(1) Data retrievals that Firefox makes on a regular basis. See: How to stop Firefox from making automatic connections

(2) New Tab Page - missing thumbnails

Every time you open the new tab page, by default Firefox prepares 15 tiles (3 rows of 5). You might need to zoom out to see all of them. If there is a site for which Firefox was not able to capture a tile, it tried to create one in the background. If you see such a site and do not need it on the page, you can hover your mouse over that tile and "x" it off.

(3) Service Workers

Some sites store local data and register a "service worker" to listen for updates in the background. This might require your permission to receive notifications in order to operate.

To view and disconnect service workers, you can view the Registered Service Workers page. Type or paste about:serviceworkers in the address bar and press Enter/Return to load it.

If you find sites listed that you have no intention of revisiting in the near future, you can click Unregister.

(4) Add-ons that connect to sites

Some are obviously, such as email notifiers. Others may be less obvious.

I can't thank you enough for this invaluable information!

It would be great if Firefox were to implement a setting so you can choose a setting that will change the about:config settings to control Firefox's pre-emptive mechanisms to suite your bandwidth/data usage needs. Something like "Intelligent Prefetch & Updating", "Updating Only" and "Legacy Browsing" where legacy browsing would do next to no pre-emptive connections/browsing.

I truly appreciate the ingenious ways that browser companies are coming up with to make ones browsing experience faster and more fluent. The problem is however that sadly in some countries data costs are just about daylight robbery and without control over ones bandwidth usage, no matter how insignificant the requests may seem, it can become very expensive very quickly. It all adds up exponentially at some point.

Between all the software companies that think they can just poll and download for updates continuesly, companies analysing your traffic for add's/profiled content delivery that you never even look at and web design that's becoming less and less responsible in terms of bandwidth/data requirements, it's becoming a real uphill struggle to keep a handle on ones data usage.

Again, thank you for your great response. I have tweaked a couple of things and know a bit more about what to watch out for now.

Regards