First, have your ISP test your service.

What were you doing at the time? surfing, watching a clip, reading something . . . .

Is this with all websites/pages, some, a few?

About how long before you lose the connection?

Start Firefox using Troubleshoot(Safe) Mode {web link}

A small dialog should appear. Click Start In Troubleshoot(Safe) Mode (not Refresh). Is the problem still there?

Hi, there is no need to ask my ISP provider to test the service because it happens only with Firefox, not another browser. After using the SafeMode, I realised that the issue comes from one of my extensions and it is only with Windows 8.1; since I have only 3 extensions it can only be either Privacy Badger, UBlock Origine or Norton SafeWeb. I will test further to determine which one it is. Thanks for the help.

It could be the work of one of your add-ons, or it could be Hardware Acceleration. How to disable Hardware Acceleration {web link}

Type about:addons<enter> in the address bar to open your Add-ons Manager. Hot key; <Control> (Mac=<Command>) <Shift> A)

In the Add-ons Manager, on the left, select Extensions. Disable a few add-ons, then Restart Firefox.

Some added toolbar and anti-virus add-ons are known to cause Firefox issues. Disable All Of them.

If the problem continues, disable some more (restarting FF). Continue until the problem is gone. After, you know what group is causing the issue. Re-enable the last group ONE AT A TIME (restarting FF) until the problem returns.

Once you think you found the problem, disable that and re-enable all the others, then restart again. Let us know who the suspect is, detective.

And the culprit is... uBlock Origin

That was very good work. Well Done. You should contact their support on this.