firefox: Update to 88.0.1

Changelog:
Version 88.0.1, first offered to Release channel users on May 5, 2021
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Fixed

  * Resolved an issue caused by a recent Widevine plugin update which prevented
    some purchased video content from playing correctly (bug 1705138)

  * Fixed corruption of videos playing on Twitter or WebRTC calls on some Gen6
    Intel graphics chipsets (bug 1708937)

  * Fixed menulists in Preferences being unreadable for users with High
    Contrast Mode enabled (bug 1706496)

  * Various stability and security fixes.

Security fixes:
#CVE-2021-29953: Universal Cross-Site Scripting
#CVE-2021-29952: Race condition in Web Render Components

(ryoon)