firefox: Update to 75.0

Changelog:
New
    With today's release, a number of improvements will help you
    search smarter, faster. Type less and find more with Firefox's
    revamped address bar:

Focused, clean search experience that's optimized for
smaller laptop screens

Top sites now appear when you select the address

Improved readability of search suggestions with a focus on
new search terms

Suggestions include solutions to common Firefox issues

On Linux, the behavior when clicking on the Address Bar
and the Search Bar now matches other desktop platforms: a
single click selects all without primary selection, a double
click selects a word, and a triple click selects all with
primary selection

    Firefox will locally cache all trusted Web PKI Certificate
    Authority certificates known to Mozilla. This will improve
    HTTPS compatibility with misconfigured web servers and improve
    security.

    Firefox is now available in Flatpak, an easier way to install
    and use Firefox on Linux.

    Direct Composition is being integrated for our users on Windows
    to help improve performance and enable our ongoing work to ship
    WebRender on Windows 10 laptops with Intel graphics cards.

Fixed
    Various security fixes

Enterprise
    Experimental support for using client certificates from the OS
    certificate store can be enabled on macOS by setting the
    preference security.osclientcerts.autoload to true.

    Enterprise policies may be used to exclude domains from being
    resolved via TRR (Trusted Recursive Resolver) using DNS over
    HTTPS.

Developer
    Developer Information

    Save bandwidth and reduce browser memory by using the loading
    attribute on the <img> element. The default "eager" value loads
    images immediately, and the "lazy" value delays loading until
    the image is within range of the viewport.

    Instant evaluation for Console expressions lets developers
    identify and fix errors more rapidly than before. As long as
    expressions typed into the Web Console are side-effect free,
    their results will be previewed while you type.

Security fixes:
#CVE-2020-6821: Uninitialized memory could be read when using the WebGL copyTexSubImage method
#CVE-2020-6822: Out of bounds write in GMPDecodeData when processing large images
#CVE-2020-6823: Malicious Extension could obtain auth codes from OAuth login flows
#CVE-2020-6824: Generated passwords may be identical on the same site between separate private browsing sessions
#CVE-2020-6825: Memory safety bugs fixed in Firefox 75 and Firefox ESR 68.7
#CVE-2020-6826: Memory safety bugs fixed in Firefox 75

(ryoon)