Rated 4 out of 5 stars

This is a nice, little, unobtrusive and helpful extension. I just installed it, but can see it will be quite helpful when I need to do a quick calculation while browsing and posting on different pages.

A few suggestions:

1) The precision does not match the display length. For instance, if I calculate "1/3" the display shows the answer "0.33333333333" (13 characters and a little bit of the fourteenth). If I copy/paste the answer, it shows "0.3333333333333333" (18 characters). If you calculate to 18 digits of precision, you should be able to see all 18.

2) You should consider rounding the last digit of the answer. "2/3" shows "0.6666666666666666" but should round to "0.6666666666666667".

3) It would be nice to have a button that copies the answer to the clipboard. This would make it easier to paste the results into a form or forum post page.

4) It seems obvious that your goal was to present a very basic calculator and you have succeeded in that. In a future release you may consider adding more functions that put it on par with a scientific calculator (square root, pi, sine, cosine, add to memory, etc.)

This review is for a previous version of the add-on (0.1.1-signed). 

Thank you for reviewing the addon. As for precision and rounding the last digit has been fix in version 1.0. Added button that copy to clipboard in the version 1.1.0. As to other suggestion to add more functions to par in scientific calculator will be considered and I hope it will be available in future release, though it is far from perfect.
Any suggestion, advice or constructive criticism, please do submit it.