Hi nazirmd, have you heard about the optional userChrome.css file, and using style rules in that file to modify Firefox's interface? You can use that method to show more rows on the Bookmarks Toolbar:

https://www.userchrome.org/what-is-userchrome-css.html#multirowbookmarks

Since you mentioned Roomy, is your main concern the size of icons on the Bookmarks Toolbar, or on all toolbars?

Elsewhere on the forum, there's an example of how to cram the main toolbar icons closer together in the following thread, but I don't think I've ever tried making the icons smaller in all dimensions.

https://support.mozilla.org/questions/1189009#answer-1037543

Maybe this reply has enough keywords to search up solutions for all of the different bits and pieces but, if not, let us know in a little more detail what changes you'd like to make.

I need to reduce the space between the icons. I thought there will be easier and quicker way... Thanks anyway.

Currently, there is not an easier or quicker way because add-ons do not have direct access to the toolbar area except for a few things, like settings colors.

If you look at the following screenshot, what do you think of that spacing?

https://prod-cdn.sumo.mozilla.net/uploads/images/2017-11-21-23-22-46-b33f83.png (lower example)

Same thing on the Bookmarks Toolbar, squeeze out extra space next to the icons?