Flash Player is still very much supported on Firefox. The 'installation failed' error is no indication that it's no longer available for Firefox.
To troubleshoot the error, I'll need to get some log files from you.
If you're using the online (shim) installer, downloaded from get.adobe.com/flashplayer. I'll need the following log files: Adobe_ADM.log, Adobe_GDE.log, FlashPlayerInstallManager.log and install.log
If you're using the full/offline installer, I'll need the following files: FlashPlayerInstallManager.log and install.log
Instructions for obtaining the log files are in the Where do I find the Flash Player installation log on the Macintosh? FAQ.
After gathering all files, upload them to cloud.acrobat.com/send using the How to share a file using Adobe Document Cloud instructions.
Thank you for posting the log files. Based on the log file, this is the online (shim) installer and is failing due to a known bug that we are working to resolve. The bug is not reproducible in the offline installer. Please try using the offline installer, posted at the bottom of the Installation problems | Flash Player | Mac page in the 'Still having problems' section.
Problem solved. Thank you!