That is the way it is designed, but it is possible to change.

You could install the Stylish add-on and then install a style which does what you want: Sidebar without size restrictions userstyle.

Or you can edit a configuration file directly, by following instructions for creating or editing the userchrome.css file. Adding the following CSS to that file should do what you want:

/* Remove width restrictions on sidebar */
#sidebar-box {
    overflow-x: hidden !important;
}
#sidebar {
    min-width: 0px !important;
    max-width: none !important;
    overflow-x: hidden !important;
}

Thank you for the information. I really do hope that Firefox software engineers watch this space. It would be very interesting to be a fly on the wall in the meeting where this bit of software design was being discussed.

It's kinda like this: " Let's see, we need to determine the default setting for our browser's bookmark sidebar width control. We can either make the minimum width about 1.5" more or less approximately just about maybe perhaps OR we can make the default 'let the user drag the width to any size desired." Can we have a show of hands on this? OK, thank you, the default will be to fix the minimum width to 1.5" more or less approximately. Next?

OK, now that we have decided to make the default a minimum width of about 1.5" more or less approximately, let's decide best default action to take wrt the default setting for the Home page. When the user sets the home page on the first tab, should the default be to make that setting the default for every tab OR ought we require the user to locate, download and install an add-in to get this functionality?

Yeppers, being a fly on the wall in THAT meeting room would have been one heck of an interesting experience.

I do not get it.

Thanks again,

Fred