Why not just bookmark https://addons.mozilla.org/en-US/firefox/ and use that? That works for me.

That can happen if you have a lot of extensions installed.

The "Get Add-ons" tab on the about:addons page loads content from the Mozilla server and that can take some time especially if you have a lot of extensions installed or the server is busy.
Firefox sends a list of installed extensions to the server to prevent extensions that you have install from appearing in the list that is displayed.
You can click on the Extensions tab to see the installed extensions or one of the other tabs to see other installed add-ons.

Thank you for the reply. Clicking on Extensions causes no action nor display of anything. The Get Add-ons icon has been showing Loading for three hours now. Before this problem recently began occurring, the number of extensions I was using was limited to only a few (less than ten perhaps).