I’ve made a Firefox extension that converts pinyin with tone numbers into pinyin with tone marks. The specifics of the conversion process are identical to those of the online pinyin converter I wrote earlier.
After installing the extension, a blue square will appear on the right side of the status bar at the bottom of your Firefox web browser. To use the tool, type some pinyin with tone numbers into any plain text field on any web page. Then highlight the text and click on the blue 拼 on your status bar. It will convert the tone numbers into the appropriate marks over the appropriate vowels.
For example, if you type in “zhong1wen2”, highlight it and hit the button, then it will be converted into “zhōngwén”.
Go to the download page to get it.