We talked about Firefox’s about:compat page in February 2019 when it first launched in the Nightly version of the web browser.
The internal page highlights compatibility tweaks that Firefox makes when it connects to certain sites.
Generally speaking, Firefox should display the majority of sites without any issues but there are cases where that is not the case.
Some developers optimize sites only for Google Chrome thanks to the massive user base. A lack of tests may lead to loading or functionality issues. Others, Google is a prime example of this, may only allow certain browsers, e.g. Chrome, by sniffing user agents of connecting devices.
Some examples: Google made the new Google Earth application Chrome exclusive and it remained exclusive for two years. Google Docs blocked Microsoft Edge from accessing it showing an unsupported browser message, and YouTube blocked Microsoft Edge users from using the new design of the site.
Mozilla implements non-standard features in Firefox for compatibility reasons. These usually come with the -webkit prefix and aim to add support for features that Firefox does not support by default because they are not standards.
Support for these features may not be enough, however, as sites may use user agent sniffing to determine compatibility.
Mozilla has to find a way to make these sites work, especially if they are popular. The organization does so using four different methods that all fall under what it calls Web Compatibility.
- HTTP user agent overrides.
- CSS injections.
User agent overrides change the information that a browser’s user agent reveals to a site. The default Firefox user agent on Windows may reveal the string Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:68.0) Gecko/20100101 Firefox/68.0 to sites indicating that it is Firefox that is connecting.
Mozilla might change that to Chrome’s user agent instead for compatibility reasons, Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/75.0.3770.100 Safari/537.36.
Firefox users who run version 68 of the browser or newer versions may load about:compat in the browser’s address bar to display the list of current modifications.
The page separates user agent overrides from interventions. You find major sites such as YouTube, Bank of America, Twitch or Discord on the list. Each listing links to the bug description on Bugzilla, and there is an option to disable the modification to test the site without it.
Mozilla launched the Web Compat website
recently in 2014. Firefox users may browse the list of bugs on the site or report a new compatibility bug to Mozilla right away. The list of reports is huge already and with Chrome gaining more and more dominance on desktop, it seems unlikely that the trend will reverse anytime soon.
Now You: Do you use a browser that is not Chrome? When was the last time you ran into compatibility issues?
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