陈丹的博客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
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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.
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Code executed in wrong order with setTimeout after alert. This changes the behavior of the onButtonClick function to make it async.
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Drop the prefixed version of visibility API. This removes mozVisibilityState and mozHidden.
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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.
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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
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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.
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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.
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Remove the last references to Actor/FrontClass. While both of these still work for add-ons, they should be considered as deprecated.
New!
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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.
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WebExtension Experiments. This is a mechanism that allows us (and you!) to prototype new WebExtensions APIs.
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