To disable the ‘silent request‘, just set value of network.http.speculative-parallel-limit in about:config to 0
1. Type about:config into the address bar (and you’ll see a bunch of variables)
2. Copy and paste network.http.speculative-parallel-limit into the search bar at the top of that page and hit Enter.
3. You’ll now just have that one line on the page. Double-click it, a box pops up, you change the value to 0, and hit OK.
And that’s it.
How to Quash Firefox’s Silent Requests | Slashdot