This is a workaround. This needs a toolbar showing text only, no symbols - the way I have my bookmarks toolbar, but probably you can move it to your menuBar if you like.
When you "customize" after installing noUn, drag the symbols from the customizeWindow to that toolbar: the dropDownArrows will show as arrows, the other ones will show as clickable text.
Now, those words take too much space, for my tastes.
........ATTENTION: before doing this you should either a) know exactly what to do - and/or b) backup the programs\mozilla\firefox\chrome folder.............
What I did was: editing the browser.dtd in
programs\mozilla\firefox\chrome\[myLocale].jar\locale\browser\
The entries to look for are:
backCmd.label (edit "back" to "B")
forwardCmd.label (edit "forward" to"F")
That is IT - the clickableTextButtons are oneLetterButtons now!

How to edit browser.dtd: install 7zip(freeware); close Firefox; navigate to the programs\mozilla\firefox\chrome\ folder; drag [myLocale].jar to the 7zip shortcut on your desktop; doubleclicks on "locale", then "browser"; rightClick on "browser.dtd", then leftClick "edit" (opens browser.dtd in notepad here, might be different for you - but then you probably do not need this howTo anyway); menu "edit"-"find", type in backCmd.label, "find", edit the entry, then do the same for forwardCmd.label; when you close notepad, click oK when prompted for saving; - next, 7zip will ask you, if it should refresh the changed browser.dtd in the archive - confirm this; close 7zip: DONE!
Run Firefox: should anything have gone wrong: close firefox, replace the chromeFolder with your backup, and try again.