Have you tried to remove the script and install (copypaste) the default one?
Also could you try to install GreaseMonkey add-on and try to install script with it (despite the weird monkey-face icon (IMG:[
invalid]
style_emoticons/default/smile.gif) )
And one more - are you using custom font IR default font in the game? (Check the in-game settings). Chances are that Monsterbation requires custom font for work properly.
Oh, and one idea more - are you using base HentaiVerse domain (HTTPS://HentaiVerse.Org) or alt (http://alt.hentaiverse.org)? Try to switch between them. Most likely it would not help, but try anyway.
Could you also try any other script? Check my signature for "Compendium" and try any other script related to HentaiVerse (smartMonsterLab, for instance).
As an ultimate solution, I may recommend you to download portable Chromium or Iron browser, install Tampermonkey on them and give script a try. By the way, IMHO, chrome-like browsers are faster for HentaiVerse than Firefox.
Well, if nothing of these will help, than I don't even know what to think.