In case one of your extensions is involved, could you test the page in Firefox's Safe Mode? That's a standard diagnostic tool to deactivate extensions and some advanced features of Firefox. In the dialog that opens, you can remove the site's cookies individually. Tools menu > Page Info > Security > 'View Cookies'.right-click and choose View Page Info > Security > 'View Cookies'.While viewing a page on the site, try either:

(2) Remove the site's cookies (save any pending work first). If you have a large hard drive, this might take a few minutes. On the Network mini-tab > Cached Web Content : 'Clear Now' You also can clear Firefox's cache completely using: 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.