When familiarising myself with a website that I was tasked with making some changes to, I wanted to View Source to help me debug, etc.
I found that in Firefox I could view the page source as expected but in IE (it was IE6 that I was running - a bit old hat these days but then again it was an old ASP website so it wasn't the only thing from the past!) View Source was disabled. This was either by right-clicking or selecting View >> Source from the menu.
Turned out to be a security setting:
The website was a secure (https) website and the default is to disable View Source for https. The setting is at:
Tools > Internet Options > Advanced tab > Security section, option Do not save encrypted pages to disk.
When checked, this option will intentionally disable View Source