Why won't firefox start?

Question

Browsers

Greetings.
Failed to launch Firefox browser this morning. Although a couple of days ago everything worked.
I have Windows 7 Ultimate SP1. When you start firefox hangs in processes for 5 seconds and disappears. In safe mode, it starts as usual.
Of the significant events over the weekend, the following happened:
- adding a new memory bar (without this bar, Firefox still does not start);
- removal of some Avast components.
Checked for viruses. Avast is silent, dr. Web found some kind of filekiller virus (Uninstall.exe file) in AppData and deleted it. After that, I uninstalled Firefox, reinstalled, but it still won't start.
The logs say the following:

Faulting application name: firefox.exe, version: 39.0.0.5659, timestamp: 0x55933a80 Faulting
module name: unknown, version: 0.0.0.0, timestamp 0x00000000
Exception code: 0xc000041d Error
offset: 0x77531221 Faulting
process ID: 0x72c Faulting
application startup time : 0x01d0c85a15f98e02 Faulting
application path: C:\Program Files (x86)\Mozilla Firefox\firefox.exe
Faulting module path: unknown
Report code: 54ced9d2-344d-11e5-ad64-902b34afdbb9

Error Container, Type 0
Event Name: APPCRASH
Response: N/
A CAB ID: 0
Problem Signature:
P1: firefox.exe
P2: 39.0.0.5659
P3: 55933a80
P4: StackHash_f4ba
P5: 0.0.0.0
P6: 00000000
P7: c000041d
P8: 77531221
P9:
P10:
Attached files:
C:\Users\admin\AppData\Local\Temp\WERF8FE.tmp.WERInternalMetadata.xml
C:\Users\admin\AppData\Local\Temp\WER53E.tmp.appcompat.txt
C:\ Users\admin\AppData\Local\Temp\WER780.tmp.hdmp
C:\Users\admin\AppData\Local\Temp\WERC61.tmp.mdmp
These files can be found here:
C:\Users\admin\AppData\Local\Microsoft\Windows\WER\ReportQueue\AppCrash_firefox.exe_e228d2ba1d5011cb27514022127a338439e1be7_cab_04570c9d
Analysis symbol:
Re-search solution: 0
Report ID: 54ced9d2-344d-11e5-ad6-
ad6

I don't know what to do anymore. A browser is needed for work, although Chrome and IE and Safari run without problems.
Somehow a month ago, Opera stopped running, but I don’t need it, I deleted it. But it can't be related.
I look forward to your help or advice.
Thank you.

Answer 1

It's Avast's fault. In some way unknown to me, it blocks Firefox from starting. Even setting exceptions did not help. Removed it, now Firefox starts.

Answer 2

By the way...
Thanks to Alexander Vitkalov I
determined why Avast is blocking Firefox.
In my case, the MusigSig vkontakte plugin was to blame.
When I turned it off, firefox and avast became friends

Answer 3

Hold Shift while loading Firefox. When you start in safe mode, disable unnecessary add-ons.
https://support.mozilla.org/en-US/kb/troubleshoot-...