I think Firefox has always kept the relative position of your selection when printing a selection. In other words, it simply leaves blank space for the header and navigation bar.

As a workaround, I often "hack" the page temporarily using a "bookmarklet" which is a snippet of script code run from the Bookmarks toolbar. This old discussion thread shows what I use, which you can try if you like: Printing selection at mozillaZine Forums.

Otherwise, maybe there is an add-on that offers a slicker solution?

Note: A faster way to add a bookmarklet is as follows.

First, copy the entire script code to the clipboard.

Next, if you are not displaying the Bookmarks Toolbar, use View > Toolbars > Bookmarks Toolbar to display it. (If you aren't displaying the classic menu bar, you can use Alt+v to open the View menu.)

Right-click on the Bookmarks Toolbar and choose New Bookmark.

Paste the code into the Location box (the second box).

Then type a useful name in the Name box (e.g., Crop4Print) and click Add.

Now, when you want to hack the page, click the button to run the script.

Still happens with Firefox 12. I wish they'd just fix it, the way it used to be in Firefox 10, without the complicated workaround suggested above.

I looked at this again yesterday for another poster and it does seem worse now: the preservation of vertical space can span multiple pages, which I believe is a new problem.

This might be easier than the bookmarklet: the add-on Nuke Anything Enhanced has the option to remove everything except your selected text from the page. Then you can print normally (i.e., without choosing the Selection bubble). I don't see a feature to undo that, so you would need to reload the page afterwards in order to continue working with it.

Screen shots: https://support.mozilla.org/en-US/questions/923294?page=2#answer-328405

An add-on "Print Edit" was recommended in the latest Firefox Newsletter. It could be worth a try.

https://addons.mozilla.org/en-US/firefox/addon/print-edit/?src=external-newsletter

In Print Edit, click what you want printed and then use Delete Except. (Using Hide Except leaves the content way down on the page.)