Firefox Add-ons Store returns HTTP 406 Not Acceptable on direct connections, but works through a proxy

Hello, I would like to report an issue with the Firefox Add-ons Store. Recently, I suddenly encountered an "HTTP 406 Not Acceptable" error when trying to access the Firefox Add-ons Store. The store is completely unusable for me right now. Here are the specific details: Issue description: I have tried accessing the store on every network available to me, including mobile data, my office wired network, and my home wired network. Unfortunately, I get the same 406 error on all of them. Timeline and context: Everything was working perfectly fine last week. I was able to search for and install a Markdown add-on without any issues. However, starting this week, it suddenly started returning the 406 error. Additional troubleshooting: I am not using a VPN or proxy. However, if I enable a proxy, the Add-ons Store works normally. There is no antivirus software or firewall blocking Firefox's network requests. Other websites load normally. Even mozilla.org works fine, but addons.mozilla.org does not. I have also tried changing to other DNS resolvers, but that did not help. Could you please help me understand why this is happening and how I can resolve it? Thank you!