I have been trying to find work arounds. Discovered that any 11-11 item that has a "preview button" opens fine. This lets me see many items that way. Every item that is directly listed under the categories when I click on the 11-11 sale banner, all open to the "page under maintenance" pop-up.
Another work around I have found is that I can search for a brand or type/name of an item. When the results list, I click the filter box that says 11-11 and then it only shows the items on the 11-11 sale. When I do this, every item opens that I click.
*Note: Over the past few hours, I have rebooted my laptop, cleared my cache, cookies, data, updated the version of FireFox I use. Turned off add blockers and even looked at any extensions that might cause problems. Logged into Aliexpress using Google. Using Google, I encounter the same problems I encounter using FireFox.
Best of my knowledge, the problem is on Aliexpress, and not on my end.