I also ran into the same problem after a clean install. This did include unhiding files under the Windows 7 x64 AppData directory tree. I repeated the problem through a second attempt after going to the extent of using regedit to strip all references to Firestorm and Secondlife.
Why not create a version that has an automated clean install option? This issue appears to be impacting multiple users. I know I will end up going back to the older version of the client and remain on it until forced to upgrade, barring an announcement for a new and improved version that addresses the installer problems.
[1348:0984][2014-04-12T11:51:34]i001: Burn v3.7.1224.0, Windows v6.1 (Build 7601: Service Pack 1), path: M:\Incoming\Phoenix-FirestormOS-Releasex64-4-6-1-40478_Setup.exe, cmdline: ''
[1348:0984][2014-04-12T11:51:34]i000: Initializing string variable 'InstallFolder' to value '[ProgramFiles64Folder]Firestorm\'
[1348:0984][2014-04-12T11:51:34]i000: Setting string variable 'WixBundleLog' to value 'C:\Users\CERRID~1\AppData\Local\Temp\Firestorm_x64_20140412115134.log'
[1348:0984][2014-04-12T11:51:34]i000: Setting string variable 'WixBundleOriginalSource' to value 'M:\Incoming\Phoenix-FirestormOS-Releasex64-4-6-1-40478_Setup.exe'
[1348:0984][2014-04-12T11:51:34]i052: Condition 'VersionNT >= v6.0' evaluates to true.
[1348:0984][2014-04-12T11:51:34]i000: Setting string variable 'WixBundleName' to value 'Firestorm x64'
[1348:0984][2014-04-12T11:51:34]i100: Detect begin, 3 packages
[1348:0984][2014-04-12T11:51:34]w120: Detected partially cached package: Setup, invalid payload: Setup, reason: 0x80070570
[1348:0984][2014-04-12T11:51:34]i101: Detected package: vcredist_x86, state: Absent, cached: None
[1348:0984][2014-04-12T11:51:34]i101: Detected package: vcredist_x64, state: Absent, cached: None
[1348:0984][2014-04-12T11:51:34]i101: Detected package: Setup, state: Absent, cached: Partial
[1348:0984][2014-04-12T11:51:34]i199: Detect complete, result: 0x0
[1348:0984][2014-04-12T11:51:36]i200: Plan begin, 3 packages, action: Install
[1348:0984][2014-04-12T11:51:36]w321: Skipping dependency registration on package with no dependency providers: vcredist_x86
[1348:0984][2014-04-12T11:51:36]i000: Setting string variable 'WixBundleLog_vcredist_x86' to value 'C:\Users\CERRID~1\AppData\Local\Temp\Firestorm_x64_20140412115134_0_vcredist_x86.log'
[1348:0984][2014-04-12T11:51:36]i000: Setting string variable 'WixBundleRollbackLog_vcredist_x86' to value 'C:\Users\CERRID~1\AppData\Local\Temp\Firestorm_x64_20140412115134_0_vcredist_x86_rollback.log'
[1348:0984][2014-04-12T11:51:36]w321: Skipping dependency registration on package with no dependency providers: vcredist_x64
[1348:0984][2014-04-12T11:51:36]i000: Setting string variable 'WixBundleLog_vcredist_x64' to value 'C:\Users\CERRID~1\AppData\Local\Temp\Firestorm_x64_20140412115134_1_vcredist_x64.log'
[1348:0984][2014-04-12T11:51:36]i000: Setting string variable 'WixBundleRollbackLog_vcredist_x64' to value 'C:\Users\CERRID~1\AppData\Local\Temp\Firestorm_x64_20140412115134_1_vcredist_x64_rollback.log'
[1348:0984][2014-04-12T11:51:36]i000: Setting string variable 'WixBundleRollbackLog_Setup' to value 'C:\Users\CERRID~1\AppData\Local\Temp\Firestorm_x64_20140412115134_2_Setup_rollback.log'
[1348:0984][2014-04-12T11:51:36]i000: Setting string variable 'WixBundleLog_Setup' to value 'C:\Users\CERRID~1\AppData\Local\Temp\Firestorm_x64_20140412115134_2_Setup.log'
[1348:0984][2014-04-12T11:51:36]i201: Planned package: vcredist_x86, state: Absent, default requested: Present, ba requested: Present, execute: Install, rollback: Uninstall, cache: Yes, uncache: No, dependency: None
[1348:0984][2014-04-12T11:51:36]i201: Planned package: vcredist_x64, state: Absent, default requested: Present, ba requested: Present, execute: Install, rollback: Uninstall, cache: Yes, uncache: No, dependency: None
[1348:0984][2014-04-12T11:51:36]i201: Planned package: Setup, state: Absent, default requested: Present, ba requested: Present, execute: Install, rollback: Uninstall, cache: Yes, uncache: No, dependency: Register
[1348:0984][2014-04-12T11:51:36]i299: Plan complete, result: 0x0
[1348:0984][2014-04-12T11:51:36]i300: Apply begin
[1348:0984][2014-04-12T11:51:36]e000: Error 0x80070020: Failed to create engine file at path: C:\Users\CERRID~1\AppData\Local\Temp\{9cbe51c5-a36b-4708-96a0-a57005458010}\.be\Phoenix-FirestormOS-Releasex64-4-6-1-40478_Setup.exe
[1348:0984][2014-04-12T11:51:36]e000: Error 0x80070020: Failed to copy engine: 'M:\Incoming\Phoenix-FirestormOS-Releasex64-4-6-1-40478_Setup.exe' to working path: C:\Users\CERRID~1\AppData\Local\Temp\{9cbe51c5-a36b-4708-96a0-a57005458010}\.be\Phoenix-FirestormOS-Releasex64-4-6-1-40478_Setup.exe
[1348:0984][2014-04-12T11:51:36]e000: Error 0x80070020: Failed to cache engine to working directory.
[1348:0984][2014-04-12T11:51:36]i399: Apply complete, result: 0x80070020, restart: None, ba requested restart: No