Three possibilities come to mind:

(1) Your Google cookies might contain some corruption. You can save your work, clear them, and log in again.

While viewing a page on google.com somewhere, 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, google.com should already be entered at the top to drill down to just their cookies. You can remove the cookies individually. Then do a "full" reload using Ctrl+Shift+r to have Google send fresh cookies.

Does it make any difference?

(2) Partially disabled cookies. Firefox's settings allow you to accept/reject cookies in general and also on a site-by-site basis and also as third party cookies (for example, accounts.google.com is a third party to www.google.com).

To check your cookie settings in general, see the first section of this article: Websites say cookies are blocked - Unblock them.

To check site-specific cookie permissions, you can open a hidden page by typing about:permissions in the address bar and pressing Enter. In the left column, if you type goog in the box that says Search Sites you can click each relevant domain and see what permission you have saved for "Set Cookies".

(3) Disabled "local storage". I'm not sure about Drive, but some sites report cookie issues when you have disabled "DOM storage" also known as "local storage". You can double-check that you haven't disabled it as follows:

(a) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(b) In the search box above the list, type or paste stor and pause while the list is filtered

(c) Scroll down and check dom.storage.enabled. If it is bolded and user set to false, double-click it to switch it back to the default value of true.

Then test to see whether that makes any difference.

Success?

Also, thank you for supplying your add-on and other configuration information. It indicates a user.js file in your current configuration. If you don't remember creating that file yourself, you may want to investigate it because sometimes malware or annoying add-ons use that file to override your preferences. This article has more info on that: How to fix preferences that won't save.

Seems to have worked. Thanks.