Safe Mode is a good diagnostic to see whether your tumblr-related add-ons are a factor. To try to work around other settings and see how Firefox runs "uncustomized" on your system, could you do a two-minute experiment?

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

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

You'll click the Create Profile button. I recommend using the default location suggested (in other words, don't click the button to choose a different folder). Then start Firefox in the new profile you created.

Does tumblr lay out correctly in the new profile?

When returning to the Profile Manager, you might be tempted to use the Delete Profile button. 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.

Or if you haven't tried this: 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) Bypass Firefox's Cache

Use Ctrl+Shift+r to reload the page fresh from the server.

Alternately, you also can clear Firefox's cache completely using:

orange Firefox button (or Tools menu) > Options > Advanced

On the Network mini-tab > Cached Web Content : "Clear Now"

If you have a large hard drive, this might take a few minutes.

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

• right-click and choose View Page Info > Security > "View Cookies"
• Alt+t (open the classic Tools menu) > Page Info > Security > "View Cookies"

In the dialog that opens, you can remove the site's cookies individually.

Then try reloading the page. Does that help?

Thank you for your time and suggestions. Unfortunately, after trying all of these, Tumblr is still not displaying properly.

The style sheets are on a server with a slightly different host name:

www.tumblr.com vs. assets.tumblr.com (or secure.assets.tumblr.com on the login page)

Are you able to load a link on that server directly? For example: https://assets.tumblr.com/assets/styles/onboarding.css

(Shows a bunch of style rules run together, don't be alarmed.)

Yes, I can load links onto the server just fine, it appears.

Ah, after refreshing the page again, the layout suddenly loaded correctly. I have no idea why it's now working, but thank you very much for your help and your time, I greatly appreciate it. :)

There was another thread where a user would get to a site that its style sheets hosted on other servers would not load. If he went into the source and found a link to the style sheet and went there directly, then it also would load into the regular page. We didn't figure out why that was, thinking maybe it was something in the security software, or maybe a third party cookie issue. Anyway, if you notice it on other sites, we should investigate further.