Mozilla has added a new feature to Firefox 39 which emulates some -webkit prefixes on select sites to improve compatibility of those sites in the Firefox browser.
One issue that you may run into when browsing sites on the Internet is that web developers may have used vendor specific prefixes for styling. While that is not an issue if alternatives have been implemented, it can cause display issues when that is not the case.
Common prefixes are webkit, used by Chromium-based browsers, moz, used by Mozilla-based browsers, and ms used by Internet Explorer.
A comparison list of supported prefixes in rendering engines is available on this page. There you will notice that some prefixes don't have equivalents in other browsers, for instance if a specific property is not supported by that browser.
If you take a look at the following example, you will notice layout issues on the site in Firefox if you are using a pre-39 version to access it and no such issues in Firefox 39 or later.
The problem here is that these layout issues may reflect badly on the web browser and that appears to be the main reason why Mozilla decided to implement webkit prefix emulations for select websites.
The emulation is hardcoded into Firefox 39 which means that it will only work on a list of sites that Mozilla added to it specifically.
The majority of these sites appear to be Japanese and Chinese, with many of them related to images and mobile devices.
All sites listed should display better in Firefox 39 and especially in Firefox for Android considering that the majority of sites listed are mobile-related.
The main reason why this is implemented only for select websites is simple: if Mozilla would enable it for all, it would send a wrong signal to the web development community.
Firefox users can turn off the emulation on the browser's about:config page:
A value of true means it is enabled, one of false that it is disabled in Firefox.
Check out bug 1107378 for additional information and implementation in the Firefox web browser.
This change won't affect Firefox users who don't visit sites supported by the workaround. It is however likely that the list of sites will be updated regularly. (via Sören Hentzschel)
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