community/firefox-esr: security upgrade to 78.15

• https://www.mozilla.org/en-US/firefox/78.14.0/releasenotes/
• https://www.mozilla.org/security/advisories/mfsa2021-39/
• https://www.mozilla.org/en-US/firefox/78.15.0/releasenotes/
• https://www.mozilla.org/en-US/security/advisories/mfsa2021-44/