Alternate method to make Firefox the default browser for local html 
files, in other words so that left click on any .html file opens in 
Firefox by default:

[1] Right click on a local .html file to get context menu

[2] Select "open with" (brings up a programs menu)

[3] Expand Internet submenu and select Firefox

[4] Check "Remember application association for this type of file"

[5] Click OK. System update will take place.

Now any local .html file will open in Firefox by left click. All file
types can be configured to open on left click in a specific app by 
this method.