It is known to those who have installed Windows 11 that Microsoft has made it much more difficult to set other third-party browsers as a default one. Microsoft may consider to increase the market share of Edge, because Chrome and Firefox has been the major competitors who have dominate the desktop browser market. Due to the online security of the users, Microsoft may need to enhance the security by preventing any malicious behaviors to modify the default apps and homepage of browsers.
But the complaints also keep raising these days that many users do not want to use Edge browser. And Mozilla Firefox may be also worried about losing the market share. So Mozilla Firefox firstly starts to fight back by providing a way to to set Firefox as default browser on Windows 11.

In Firefox 89 and above version, users can directly change the settings on Firefox.
Step 1: Check Firefox versions by clicking drop down menu on the right corner to select Help;

Click About to check the version you have

If your Firefox version is above 89 and above, you can set it as default in the following steps.

Step 2: Save your works and restart Firefox, then you will get the pop asking you to set as default browser

Then you can have a try to open a link to check whether it has been directly via Firefox.
Below the picture is what I get and tested successfully.

This is really a good news to the Firefox fans without any further steps to change every item such as .html, .pdf, .shtml, .svg, .webp, .xht, FTP, HTML, HTTPS one by one.
Of course, we’re still waiting for whether there is any progress on Google Chrome to do that. If you‘re a fan of Chrome, you still need to set it step by step to make it as default on Windows currently.