If it's a long text article, I usually do a copy and paste into Word, which just takes a couple of seconds. Or (from Firefox) do a "Save - Web Page Complete" then open it in Word later and save it as an RTF.
If you're hitting the same site for content, check out RSS2Book:
https://www.mobileread.com/forums/showthread.php?t=7946
Your idea for a Firefox extension is great. Particularly if it was optimized for the Reader (e.g. letting you preset the font, size, etc. and giving you a box for title/author that pops up for during the save).