Firefox will not open from the start menu or the taskbar, but WILL open when called from Windows Live Messenger

Question

This started a couple weeks ago, but doesn't seem to coincide with any other installations or reconfigurations, so I'm having trouble getting at the root cause.
The first time I run Firefox after installing, it runs fine (complete with a full list of add-ons.) If I then close FF and try to start it from the task bar or start menu, nothing happens. I've tried starting in safe mode (shift+FF), but get the same result. I've restored FF to all the default settings, disabled all my plugins and add ons, but no change.
The really odd part is that if I click the 'Inbox' link in my Windows Live Messenger, FF opens perfectly. This made me think that it was my homepage causing the issue, but A: I can go to that page without problems once FF has started from WLM, and B: even if I use the FF default homepage or google.com, I get the same problem. Furthermore, if I restart FF from within FF (like after I disable/enable an add-on), it will also start properly.
No crash report is generated by FF when it fails to load, but Windows catches the problem in it's event log. I've pasted the contents of that record at the bottom in the hopes that someone can take a stab at what's wrong.
Finally, I've already run through the online crash help, but everything is either not related (since FF will start if called indirectly) or has already been prevented (absolutely no add-ons are running at the moment.)
Event View Crash Report:
Log Name: Application
Source: Application Error
Date: 06/06/2011 7:29:50 PM
Event ID: 1000
Task Category: (100)
Level: Error
Keywords: Classic
User: N/A
Computer: UberHal
Description:
Faulting application name: firefox.exe, version: 2.0.1.4120, time stamp: 0x4da6a9fb
Faulting module name: unknown, version: 0.0.0.0, time stamp: 0x00000000
Exception code: 0xc0000005
Fault offset: 0x8d394fd6
Faulting process id: 0x1aec
Faulting application start time: 0x01cc24a1a155376a
Faulting application path: C:\Program Files (x86)\Mozilla Firefox\firefox.exe
Faulting module path: unknown
Report Id: df4396d9-9094-11e0-8761-0026180e006b
Event Xml:
<Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
<System>
<Provider Name="Application Error" />
<EventID Qualifiers="0">1000</EventID>
<Level>2</Level>
<Task>100</Task>
<Keywords>0x80000000000000</Keywords>
<TimeCreated SystemTime="2011-06-06T23:29:50.000000000Z" />
<EventRecordID>31380</EventRecordID>
<Channel>Application</Channel>
<Computer>UberHal</Computer>
<Security />
</System>
<EventData>
<Data>firefox.exe</Data>
<Data>2.0.1.4120</Data>
<Data>4da6a9fb</Data>
<Data>unknown</Data>
<Data>0.0.0.0</Data>
<Data>00000000</Data>
<Data>c0000005</Data>
<Data>8d394fd6</Data>
<Data>1aec</Data>
<Data>01cc24a1a155376a</Data>
<Data>C:\Program Files (x86)\Mozilla Firefox\firefox.exe</Data>
<Data>unknown</Data>
<Data>df4396d9-9094-11e0-8761-0026180e006b</Data>
</EventData>
</Event>
Log Name: Application
Source: Windows Error Reporting
Date: 06/06/2011 7:29:56 PM
Event ID: 1001
Task Category: None
Level: Information
Keywords: Classic
User: N/A
Computer: UberHal
Description:
Fault bucket 2009598108, type 5
Event Name: BEX
Response: Not available
Cab Id: 0
Problem signature:
P1: firefox.exe
P2: 2.0.1.4120
P3: 4da6a9fb
P4: StackHash_0a9e
P5: 0.0.0.0
P6: 00000000
P7: 8d394fd6
P8: c0000005
P9: 00000008
P10:
Attached files:
C:\Users\MC\AppData\Local\Temp\WERD47E.tmp.WERInternalMetadata.xml
These files may be available here:
C:\Users\MC\AppData\Local\Microsoft\Windows\WER\ReportArchive\AppCrash_firefox.exe_966114b87f1ef26bf245bee89f49ab1bb58152f2_0c65eb58
Analysis symbol:
Rechecking for solution: 0
Report Id: df4396d9-9094-11e0-8761-0026180e006b
Report Status: 0
Event Xml:
<Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
<System>
<Provider Name="Windows Error Reporting" />
<EventID Qualifiers="0">1001</EventID>
<Level>4</Level>
<Task>0</Task>
<Keywords>0x80000000000000</Keywords>
<TimeCreated SystemTime="2011-06-06T23:29:56.000000000Z" />
<EventRecordID>31381</EventRecordID>
<Channel>Application</Channel>
<Computer>UberHal</Computer>
<Security />
</System>
<EventData>
<Data>2009598108</Data>
<Data>5</Data>
<Data>BEX</Data>
<Data>Not available</Data>
<Data>0</Data>
<Data>firefox.exe</Data>
<Data>2.0.1.4120</Data>
<Data>4da6a9fb</Data>
<Data>StackHash_0a9e</Data>
<Data>0.0.0.0</Data>
<Data>00000000</Data>
<Data>8d394fd6</Data>
<Data>c0000005</Data>
<Data>00000008</Data>
<Data>
</Data>
<Data>
C:\Users\MC\AppData\Local\Temp\WERD47E.tmp.WERInternalMetadata.xml</Data>
<Data>C:\Users\MC\AppData\Local\Microsoft\Windows\WER\ReportArchive\AppCrash_firefox.exe_966114b87f1ef26bf245bee89f49ab1bb58152f2_0c65eb58</Data>
<Data>
</Data>
<Data>0</Data>
<Data>df4396d9-9094-11e0-8761-0026180e006b</Data>
<Data>0</Data>
</EventData>
</Event>

Answer 1

I have exactly the same issue. After FF upgraded on its own to version 5, it won't open from start menu, taskbar, desktop icon, or even by pressing firefox.exe in program files directory. It does so by pressing a link at msn messenger, inside Outlook etc.
Disabling all addons, didn't help. Moreover, I can't start FF in safe mode or start profile manager (Start > Run> firefox.exe -ProfileManager).
OS: Windows 7 64bit
Windows Event Viewer entry details:
Faulting application name: firefox.exe, version: 5.0.0.4183, time stamp: 0x4df95302
Faulting module name: unknown, version: 0.0.0.0, time stamp: 0x00000000
Exception code: 0xc0000005
Fault offset: 0x8c724fa6
Faulting process id: 0xc0c
Faulting application start time: 0x01cc37fe2fae7e8b
Faulting application path: C:\Program Files (x86)\Mozilla Firefox\firefox.exe
Faulting module path: unknown
Report Id: 6e7f03eb-a3f1-11e0-8f4d-a4badb0238f6