I'm sorry to hear about being on Manjaro.

Just kidding, but try downloading Firefox from Mozilla. Download, unzip, and run firefox-bin from the folder and see if you have the same issue. https://www.mozilla.org/en-US/firefox/all/#product-desktop-release

Do you have the same issue with Firefox in Troubleshoot Mode? https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode#w_how-to-start-firefox-in-4troubleshoot-modesf5safe-modesf

Troubleshoot mode also has same issue. And firefox-bin opens the addons page but does not install.

What desktop? X11 or Wayland? What are you trying to install? Provide a link to the add-on. Also see if you have the same issue under a different user account and maybe try your OS on a live usb stick.

I am on a X11 desktop. I want Grammarly and Mendeley web importer extensions. It's not these two specifically, I can not install any addon. And the same issue persists with different account. I tried resetting, this also did not help.

It is like the add-on site is blocked. It just does not open. Always ends up in 'The connection has timed out'.

I installed the extensions in my macbook's firefox and sync the settings in my linux firefox. This downloaded and installed them. Kind of a workaround.

What desktop? May also need to check in on the manjoro forum to see if others are having the same issue. Who are you using for DNS? Post a screenshot of the page.

I am connected to my institute's network. I don't know the DNS. But the other systems on the same network don't have this issue. I installed the same version of Manjaro on other system (same hardware configs), It is working fine. I am thinking of reinstalling OS.

I would try your OS or another... wink wink... on a live usb stick and see if you have the same issue.

I have tried that. I have debian and also manjaro. No problem there. PS: I will surely try openSUSE. ;)