Yes, you'll be able to use everything except Sync. However, it's also recommended to create bookmarks' backup once in a while, see https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer#firefox:mac:fx125

Firefox 78.15.0esr is the latest to still support the old EOL macOS 10.9, 10.10, 10.11 https://support.mozilla.org/en-US/kb/firefox-mac-osx-users-esr Firefox 79.0 to 115 ESR requires macOS 10.12, 10.13, 10.14 at minimum to run. https://support.mozilla.org/en-US/kb/firefox-users-macos-1012-1013-1014-moving-to-extended-support Firefox 116.0 and later requires macOS 10.15 or later to run. If you can update to at least macOS 10.12 (ten.twelve) then you can use the still currently supported Fx 115 ESR (115.8.0esr) as the last update 115..15.0esr will be in Sept 2024. https://www.mozilla.org/firefox/all/#product-desktop-esr (<requires macOS 10.12 or later as it will not run on your 10.11) edit to add: The email was just stating Sync was not going to be supported anymore in the old Firefox 78.0 and earlier versions as you need 79.0 or later to use Sync. https://support.mozilla.org/en-US/kb/sync however you need macOS 10.12 or newr to use a newer Firefox version.