Problem occurs when trying to upload a list more than 10,000 leads using the Firefox browser. To fix this issue, you need to change the default value of the response time set from 300 seconds to 1800.
Warning: Modifying preferences can, in rare circumstances, break Firefox, Thunderbird or the Mozilla Suite, or can cause strange behavior.
Step 1. Launch your Firefox browser and then, open a new tab.
Step 2. Typed in the address bar the about :config and then, press Enter.
Step 3. Once the Warning Page appears, just click on button that say "I'll be careful, I promise!"
Step 4. From the Search bar, type the word response and look for the network.http.response.timeout in the list.
Step 5. Double-click the network.http.response.timeout and change the value from 300 to 1800. After that, click on the OK button to apply the changes.
Step 6. Once done, it should show now the 1800 for the value. Just close the browser to close the window.