I usually open multiple related/relevant tabs about a topic I search on; and after having finished reading one of the tabs, each after, I decide to whether bookmark it or not.
Sometimes depending on the relavance to my search or the content's usefulness, I choose to saving that bookmark with a set of particular tag(s). But, when I have to do this action for more than once for those multiple tabs I have opened, It quickly turns into a tedious task; and a solution to this which would not hurt usability and without any drawbacks to how things are now and that in some way made the process faster (I thought to myself) is to make "Tags" entry —in "Add/Edit Bookmark" dialog— preselected by default so that either if you want to edit or add new tags: you can just type characters just like how you can currently do, and typed characters will overwrite recent tags from the previous entry that was highlighted/preselecter or if you want to quickly reuse the recents tag(s) from the previous entry: you just hit <Return>or click on "Save" button to add a new bookmark with previous recent tag(s) without the hassle to having to write them all over again.
Here is an example:
Suppose you search on the net for "What is managed code?", and from the sites you have come across you open four tabs three of which is dealing on specifics of managed code with examples from C# and the other is a generic explanation on managed code. So after having read them all and each after having finished once in all tabs, I, in the end, chose to save as bookmark: 2 tabs that were detailing with C# and the other tab which explained the topic in general into a folder called "Managed Code". I also added tag(s): "C#, ManagedCode" to tabs with C# specifics, and just the tag "ManagedCode" to the tab explained the topic more generally. Now, as you may know, Firefox retains your recently chosen bookmark folder, so while I was bookmarking those three tabs, I didn't need to select for each of them the folder I wanted to save them into. On the contrary, each time I wanted to enter tags for the tabs, I was faced with this view:
But this view is what I suggest to be added (and that which would ease out adding same tags over and over to multiple consecutive bookmarks):