[Fix] Firefox 100 or Later Versions Having Issues with Some Websites

If you installed or upgraded to Mozilla Firefox 100 or later version and some websites are having browsing issues with the new version of Firefox, this article will help you in fixing the issue and opening the broken websites in Firefox 100+ versions.

Recently Mozilla released version 100 of its popular web browser “Firefox”. Since the version number of Firefox has been increased from 2 digits to 3 digits, the Useragent string (UA) used by the browser now also contains a 3-digit number instead of the previous 2-digit number.

Useragent is a string of alphabets and numbers which is used by web browsers so that servers can identify them. For example, following is the useragent string used by Firefox older versions:

Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:99.0) Gecko/20100101 Firefox/99.0

You can see the above mentioned useragent string tells servers that the used Firefox version is 99.

Now the newer versions of Firefox such as version 100 will use following useragent string:

Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:100.0) Gecko/20100101 Firefox/100.0

You see the version number has been increased from 2-digit to 3-digit in the UA string and this is the thing which can prevent some websites from opening or working in Firefox 100 and later versions.

You can check the installed Firefox useragent string using abut:support URL in the address bar or using select Help -> More troubleshooting information option from Firefox main menu as shown in following screenshot:

If a website server software or an app was not designed or upgraded to understand 3-digit browser versions in the Useragent string, the app or website might become broken in the 3-digit browser versions. Either the website will not open and display error messages or might not work properly and you’ll face lots of browsing issues while visiting the website.

The same thing happened when Google Chrome and Microsoft Edge browsers reached to version 100. To handle similar websites compatibility issues, both browsers provided solutions or workarounds to fix broken websites. You can find the details in following article:

[Fix] Apps or Scripts are Broken in Chrome / Edge 100 and Later Versions

Mozilla team has also provided similar method to fix incompatible websites issues in Firefox 100 and later versions.

If you are also facing issues with some websites after upgrading to Firefox 100+ version, following method will help you in fixing the issue:

1. Open Firefox and type about:config in the addressbar and press Enter. It’ll show you a warning message, click on “Accept the Risk and Continue” button. It’ll open Firefox’s hidden secret advanced configuration page i.e. about:config page.

2. Now type forceversion in Search filter box and look for following preference in the window:

The preference value is set to 0 by default, which means the preference is disabled.

3. To force Firefox to use 2-digit version in useragent string, double-click on network.http.useragent.forceVersion preference and set it to 99

Restart Firefox and try to open the problematic website. Now the website will work fine in Firefox.

If setting the preference value to 99 doesn’t fix the issue, try to set the value to other version numbers such as 98, 90, etc.

PS: To restore default useragent string in Firefox, set the above mentioned preference to 0 again.

Also Check:

[Tip] Restore Full Classic Scrollbars in Mozilla Firefox on Windows 11

[Tip] Restore Classic Download Confirmation Prompt in Mozilla Firefox