Rated 4 out of 5 stars

Here is the reply from IDM support:

We have tested it more closely.

Indeed if you not simply restart browser but close and open again, such problem persists.

But IDM add-on does not have code that deletes elements from popup menu, even for its own elements. But Column
Reader has several times deletion of some elements from menu, and it uses some complex ID of elements, so they
could easily mix something.

We are sure that this but is only from Column Reader, you should contact their support.

This review is for a previous version of the add-on (5.2).  This user has 2 previous reviews of this add-on.

I recently encountered a similar problem in the ThunderBird mail program. The problem would be present only one in three times after start-up, but that was sufficient for fixing it. In ThunderBird, the reason for failure was that the XUL overlay code still isn't compatible to bootstrapped extensions, even though these have been around for six years now. Merely adding an event handler to the content context menu at an early stage of program start-up would prevent other items from being added to this menu.

I have never encountered this problem in Firefox, but I think it likely has the same cause when it occurs. The fix which worked in ThunderBird will be provided in the next update of Column Reader.