Cannot open Firefox browser.

[nitin]

The Application (Firefox) may not open while opening from its shortcut stored on the desktop or even by executing the application file "firefox.exe" from the default installed directory.

In some cases, it may also display error as "Windows cannot find 'C:\Program files\Mozilla Firefox\Firefox.exe'

Solution:
A malicious software may prevent the user from runnig such applications (For example: - the Internet Explorer and Firefox browsers, the command shell "CMD.EXE").
It creates the "Debugger" string in order to prevent the programs indicated in the values from being run:

HKEY_LOCAL_MACHINE\ SOFTWARE\ Microsoft\ Windows NT\ CurrentVersion\ Image File Execution Options\ cmd.exe
Debugger
- This refers to the command shell (CMD).

HKEY_LOCAL_MACHINE\ SOFTWARE\ Microsoft\ Windows NT\ CurrentVersion\ Image File Execution Options\ firefox.exe
Debugger
HKEY_LOCAL_MACHINE\ SOFTWARE\ Microsoft\ Windows NT\ CurrentVersion\ Image File Execution Options\ iexplore.exe
Debugger
-These two entries belong to the Firefox and Internet Explorer browsers.

So, simply delete the "Debugger" String itself or delete the entire value data of "Debugger" string and then you can open that particular application without any problem.

Do note that you will not find the applications executables name under the below said registry path on a working system. It will be present only on problematic systems.
-HKEY_LOCAL_MACHINE\ SOFTWARE\ Microsoft\ Windows NT\ CurrentVersion\ Image File Execution Options