fcitx-sogoupinyin is currently the best choice,
and ibus-sunpinyin is another good alternative.
Chrome Extension
Firefox Extension
http://toshuo.com/chinese-tools/pinyin-input-firefox-extension/
Ref
http://www.pinyinput.com/?lang=en
Use Sogou Pinyin to Type Chinese
Things on this page are fragmentary and immature notes/thoughts of the author. Please read with your own judgement!
Settings
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常用 -> 隐藏状态栏
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use half width punctuation
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按键 -> 切换英文状态时，保留输入框中已存在的 …