We recommend not using 3rd party programs to clean Firefox as the wrong data can be corrupted.

Start Firefox in Safe Mode {web link}

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

Thanks for your recommendation. Before I received your message, I deselected and later reselected certain inessential data in Firefox Sync. Everything seems working fine now. I'm not sure whether it is related. In case the issue returns, I will try Safe Mode and let you know the results.

One day later and the issue came back. Unfortunately, I still was not able to identify cause of it, although I have some more findings.

I noticed that the loading halts with these external addresses in the status bar: secure[DOT]adnxs[DOT]com, ads[DOT]pubmatic[DOT]com, track[DOT]adform[DOT]net, ues[DOT]rubiconproject[DOT]com or spir[DOT]hit[DOT]gemius[DOT]pl.

I tried to run Safe Mode and the issue was still present. I updated Adobe Flash and even disabled it later. I also deleted prefs[DOT]js in Firefox profiles folder. None of it made the issue go away. Signing off from Firefox Sync and deleting inessential user data via CCleaner fixed the issue, but I'm afraid it is just temporary.

I understand you do not recomment to use 3rd party tools to clean Firefox, but it seems like it is the only thing that helps to make Firefox work fine for at least few hours.

I'm willing to delete all my Firefox Sync data, restore most essential user data from my offline backup and sing in to Firefox Sync again to make a new record in the cloud, but it will take some time and I'm afraid the issue will come back.

Any other ideas? Thanks.

I called for more help. For now, leave sync off.

Ok, so I undergone the procedure I was talking about: deleting my Firefox Sync Account, getting rid of everything Firefox related from my system drive, making clean Firefox installation and restoring essential user data from my offline backup, namely bookmarks (places.sqlite) and passwords (key4.db and logins.json). Then I set up everything by hand as I had it before, which took around two hours. A day later, even before I created a new Sync account, the issue came back.

What am I suppose to do now? Thanks.

Boot the computer in Windows Safe mode with network support to see if that has effect in case security software is causing problems.

• https://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• https://www.7tutorials.com/4-ways-boot-safe-mode-windows-10

What data are you including with Sync (bookmarks, history) ?

Did you restore history via Sync ?

As i said previously, the issue occurs even with Sync disabled. It turned out it has something to do with cookies. When I deleted cookies via CCleaner, the issue persisted, but now when I use build-in Firefox tool for that, the issue goes away for a while. After a day or so of browsing, the issue unfortunately always come back.

The issue came back today and deleting all cookies with Firefox build-in tool in Settings solved it, so I have to confirm what I was saying in my previous post. Most affected sites are csfd .cz and mobilmania .cz

I'm not using any additional security software on top of what Windows 10 Pro contains, but tried Safe Mode nevertheless. The issue was also present.

I also tried to set Enhanced Tracking Protection from Standard to Custom, with all items deselected. The issue came back as before.

Since it turned out the issue is not bind with Sync, change the title of this thread to "Certain cookies prevents various pages to load on the first try" please. I cannot rename it, since I deleted my Firefox account and created a new one.

Any other suggestions?