To test how Firefox runs "uncustomized" on your system, could you try this:

Create a new Firefox profile

A new profile will have your system-installed plugins (e.g., Flash) and extensions (e.g., security suite toolbars), but no themes, other extensions, or other customizations. It also should have completely fresh settings databases and a fresh cache folder.

Exit Firefox and start up in the Profile Manager using Start > search box (or Run):

firefox.exe -P

Don't delete anything...

Any time you want to switch profiles, exit Firefox and return to this dialog.

Click the Create Profile button, enter a name like TEST, and skip the option to choose a different folder location. After saving TEST, start Firefox in the new profile.

Does Tumblr work any better in the new profile?

When returning to the Profile Manager, you might be tempted to use the Delete Profile button on TEST. But... it's a bit too easy to accidentally delete your "real" profile, so I recommend resisting the temptation. If you do want to clean up later, I suggest making a backup of all your profiles first in case something were to go wrong.

If clearing cookies didn't help then it is possible that the cookies.sqlite file that stores the cookies got corrupted.

• Rename (or delete) cookies.sqlite (cookies.sqlite.old) and delete other present cookies files like cookies.sqlite-journal in the Firefox profile folder in case the file cookies.sqlite got corrupted.

You can use this button to go to the currently used Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)

You can check if it works in a Private Browsing mode window.

You can remove all data stored in Firefox from a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History > Show All History" or "View > Sidebar > History") or via the about:permissions page.

Using "Forget About This Site" will remove all data stored in Firefox from that domain like bookmarks, cookies, passwords, cache, history, and exceptions, so be cautious and if you have a password or other data from that domain that you do not want to lose then make sure to backup this data or make a note.

You can't recover from this 'forget' unless you have a backup of the involved files.

It doesn't have any lasting effect, so if you revisit such a 'forgotten' website then data from that website will be saved once again.

Do a malware check with several malware scanning programs on the Windows computer. Please scan with all programs because each program detects different malware. All these programs have free versions.

Make sure that you update each program to get the latest version of their databases before doing a scan.

• Malwarebytes' Anti-Malware:
  http://www.malwarebytes.org/mbam.php
• AdwCleaner:
  http://www.bleepingcomputer.com/download/adwcleaner/
  http://www.softpedia.com/get/Antivirus/Removal-Tools/AdwCleaner.shtml
• SuperAntispyware:
  http://www.superantispyware.com/
• Microsoft Safety Scanner:
  http://www.microsoft.com/security/scanner/en-us/default.aspx
• Windows Defender:
  http://windows.microsoft.com/en-us/windows/using-defender
• Spybot Search & Destroy:
  http://www.safer-networking.org/en/index.html
• Kasperky Free Security Scan:
  http://www.kaspersky.com/security-scan

You can also do a check for a rootkit infection with TDSSKiller.

• Anti-rootkit utility TDSSKiller:
  http://support.kaspersky.com/5350?el=88446

See also:

• "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked

@jscher2000, thanks for the suggestion. I tried it and the sites still did the same thing unfortunately.

@cor-el, thank you. I tried your suggestions and deleting cookies did not change anything, nor did forgetting the site.

In private browsing, I received a different error: "Content Encoding Error The page you are trying to view cannot be shown because it uses an invalid or unsupported form of compression."

Also, it is a brand new computer running antivirus, so I doubt there are any malware/spyware/virus issues yet. I did turn off firewalls just in case. Still no change.

I tried again on Chrome and noticed it gave me an error if I was not running Hola (UK or US VPN/proxy). So only when I am browsing with Hola active, I can access these sites. So I installed the FF Hola extension and again, every tumblr blog works fine as long as I am using Hola.

I can't for the life of me understand why I must do this to use tumblr on FF. I'm in Canada. My roommate's computer and my old Macbook load tumblr just fine and we're on the same network and connection. I'm starting to think it may be a problem with my computer itself... but I have no idea why.

As an update, it just started working again for no apparent reason. I haven't done anything else (except trying to fix my battery charging issue by uninstalling battery drivers).

The FF errors are fixed now but I don't know how or why.

UPDATE: Randomly started doing this again an hour later. At my wits end, I wondered about AVG (I've never had it before), and voilà. Disabled the safe web browsing option, and everything in FF works again, no proxy/VPN needed.

So I don't know if this is some incompatibility issue with AVG & FF, but this seems to have solved it. Just in case anyone else encounters this problem and all the above suggestions don't work.