We see most of time that Mozilla Firefox do not allow Internet Download manger due to security issue and that browser permanent disable IDm. So you can install some other application shows that option again.
For showing Download option there are two methods
First just install latest version of internet manger and Mozilla Firefox and then resolve your issues.
And if you don't install latest version of Mozilla Firefox and IDM then just follow these step
1. First Go to Tools in Mozilla Firefox
2. Click on the Add-Ons by short cut Combination of the keys Ctrl+shift+A
3. Now option another page Add-Ons Manger page automatically when you click Add-on options and here you can see these options
Click on option 2.Extension and write you-tube down-loader in Search bar in that page of extension then you see many application which are ready to install then its depend on yourself which you want to download. Just simply click and install any down-loader and after installing restart your Mozilla Firefox and for checking type in address bar you-tube.com and watch video then you can see Download option also.
After installing IDM in your PC, and Integrate Internet Download Manager with Firefox. IDM give you a "Download This Video" button pops up whenever you are watching a video. Simply click on the button to start downloading your desired youtube video.