Here's the general advice first:

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 and logging in again. Does that help?

Is the page you want to view a PDF? You may be able to tell this from the address bar. If it is a PDF, there could be a glitch with either the built-in PDF Viewer or the Adobe PDF viewer plugin. To try switching between then, you can use the Options dialog as described in this article: View and edit PDF files using Firefox’s built-in viewer.