Mozilla今日将Mozilla Firefox 51 Beta 1送上官方FTP，不过官网目前还并没有公布更新列表，据开发者网站介绍，51版本开始引入WebExtensions，这将使开发人员更容易将谷歌Chrome浏览器扩展移植到Firefox浏览器。Firefox曾经统治了插件的市场，但那主要是因为浏览器的市场份额萎缩了。而现在浏览器市场竞争很激烈，Firefox市场占有率有所年下降。
下载地址:
https://ftp.mozilla.org/pub/firefox/releases/51.0b1/
更新列表:
General
Let accel-click and middle-click on the new tab button open a new tab next to the current one. This changes the behavior of the new tab button and removes the BrowserOpenNewTabOrWindow function.
Code executed in wrong order with setTimeout after alert. This changes the behavior of the onButtonClick function to make it async.
Drop the prefixed version of visibility API. This removes mozVisibilityState and mozHidden.
Prevent installation of an extension when “*” is used as part of strict_min_version in a WebExtension manifest.json). Minimum versions should always be specific versions, rather than a mask.
Loaded-as-data XML documents should not generate <parsererror>. Some add-ons are relying on this node showing up if an XML document fails to parse.
XPCOM and Modules
Default to the NullPrincipal rather than SystemPrincipal for Favicons. This changes the functions setAndFetchFaviconForPage and replaceFaviconDataFromDataURL. The principal should generally be passed as an argument.
Upload setting wrong Content-Type for files if you downloaded from a server that provided the wrong Content-Type. As explained in comment #36, the way add-ons change the MIME type mappings is now different, via a category rather than directly changing mimeTypes.rdf.
Remove the last references to Actor/FrontClass. While both of these still work for add-ons, they should be considered as deprecated.
New!
You can now embed a WebExtension into any restartless add-on, allowing
you to gradually migrate your code to the new platform, or transition
any data you store to a format that works with WebExtensions.
WebExtension Experiments. This is a mechanism that allows us (and you!) to prototype new WebExtensions APIs.