Never mind, uninstalled Firefox, deleted everything browser related from disk and registry and then reinstalled.

It now works fine, I assume some corruption in the settings somewhere (this instance was an upgrade to quantum).

Removed my solution as it worked for a day then started doing the same thing.

I have the same problem, but with different web sites, namely MS Office (email), Google, and Quick Books Online.

The exact message I get with each is: "Corrupted Content Error

The site at https://outlook.office.com/owa/?realm=****.onmicrosoft.com&exsvurl=1&ll-cc=2057&modurl=0&bO=1 has experienced a network protocol violation that cannot be repaired.

The page you are trying to view cannot be shown because an error in the data transmission was detected.

   Please contact the web site owners to inform them of this problem."

Hi mollcons, do you use any filtering software or proxy server that may be intercepting and possibly modifying your website requests/responses?

Common security software that may function as a "man in the middle" include Avast, Bitdefender, ESET, Kaspersky, and Microsoft Family Safety.

More generally, if a site is generally known to work in Firefox, these are general suggestions to try when it stops working normally:

Cache and Cookies: When you have a problem with one particular site, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site.

(1) Clear Firefox's Cache

See: How to clear the Firefox cache

If you have a large hard drive, this might take a few minutes. If you do not see the number going down on the page, you can reload it using Ctrl+r to check progress.

(2) Remove the site's cookies (save any pending work first). While viewing a page on the site, try either:

• right-click (on Mac Ctrl+click) a blank area of the page and choose View Page Info > Security > "View Cookies"
• (menu bar) Tools > Page Info > Security > "View Cookies"
• click the padlock or "i" icon in the address bar, then the ">" button, then More Information, and finally the "View Cookies" button

In the dialog that opens, the current site should be pre-filled in the search box at the top of the dialog so you can remove that site's cookies individually.

Then try reloading the page. Does that help?

Testing in Firefox's Safe Mode: In Safe Mode, Firefox temporarily deactivates extensions, hardware acceleration, and some other advanced features to help you assess whether these are causing the problem.

If Firefox is not running: Hold down the Shift key when starting Firefox. (On Mac, hold down the option/alt key instead of the Shift key.)

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" Help button > Restart with Add-ons Disabled
• (menu bar) Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any improvement?

Cleared the cache - no difference.

I have a similar problem with all the other browsers on this PC (Chrome, IE, and Edge).

I do use BitDefender as my AV provider and this is hosted on their central resource.

I started Firefox (v58) in safe mode and experience exactly the same issues with the sites mentioned.

Hi mollcons, Bitdefender is one of the programs that intercepts browsing connections for filtering. As a temporary workaround, you might be able to turn off the filtering. That may be covered in this support article: How to troubleshoot security error codes on secure websites.

Don't have Bitdefender or anything similar.

However, although clearing the Cache didn't work clearing all Site Data did (temporarily).

This is not ideal as I tend to not clear caches using Firefox but use CCleaner with some cookies reserved to clear down most stuff while keeping some of my most used website settings intact

BitDefender suggested turning off Scan SSL and this has worked a treat. However, I suspect that if I turn this back on, it will recreate the issue. I am waiting for BitDefender's response to this question.

BitDefender made the following suggestions: Please do the following steps on the affected machine: 1. Access the certificates list for the browser (for example in Firefox, this is found in Options>Advanced->Certificates). Check if Bitdefender's entry is present there. 2. If it is present, delete it. If not, skip to the next step. 3. Boot the machine in safe mode 4. Delete the files from the following folders C:\Program Files\Bitdefender\Endpoint Security\mitm_cache C:\Program Files\Bitdefender\Endpoint Security\mitm_cache\cache 5. Boot the machine back in normal mode, bring up a CMD window, with admin rights, and run the command: C:\Program Files\Bitdefender\Endpoint Security\mitm_install_tool.exe /install_silent Net-Defender

I had to amend step 1 to cover all browsers by doing the following: Open Control Panel Search for User Certificates Opened Certificate Manager Opened the object type Personal >> Certificates Deleted all certificates i did not want Closed Cert Manager and Control Panel

This additional solution was contained in a Microsoft Technet forum.

When rebooted, everything seems to work fine.