firefox: Update version from 49.0.2 to 50.0

Release notes for 50.0 [1]:

> Version 50.0, first offered to Release channel users on November 15,
> 2016
>
> ### new ###
> *   Playback video on more sites without plugins with WebM EME Support
> for Widevine on Windows and Mac
>
> *   Improved performance for SDK extensions or extensions using the SDK
> module loader
>
> *   Added download protection for a large number of executable file
> types on Windows, Mac and Linux
>
> *   Increased availability of WebGL to more than 98 percent of users on
> Windows 7 and newer
>
> *   Added Guarani (gn) locale
>
> *   Added option to Find in page that allows users to limit search to
> whole words only
>
> *   Updates to keyboard shortcuts
>     *   Set a preference to have Ctrl+Tab cycle through tabs in recently
>     used order
>     *   View a page in Reader Mode by using Ctrl+Alt+R (command+alt+r on
>     Mac)
>
> ### fixed ###
>
> *   Login cookies are now saved for sites with a high number of cookies
> (Bug 1264192)
>
> *   Various security fixes
>
> *   Fixed rendering of dashed and dotted borders with rounded corners
> (border-radius)
>
> ### Changed ###
>
> *   Blocked versions of libavcodec older than 54.35.1
>
> *   Added a built-in Emoji set for operating systems without native
> Emoji fonts (Windows 8.0 and lower and Linux)

[1] https://www.mozilla.org/en-US/firefox/50.0/releasenotes/

firefox.be0

@@ -2,7 +2,7 @@
 
 #export BEE_TMP_TMPDIR=/dev/shm BEE_TMP_BUILDROOT=/dev/shm/bee-root BEE_MAKEFLAGS='-j80'
 
-# BEE_VERSION firefox-49.0.2-0
+# BEE_VERSION firefox-50.0-0
 
 SRCURL[0]="https://ftp.mozilla.org/pub/firefox/releases/${PKGVERSION}/source/firefox-${PKGVERSION}.source.tar.xz"