Click SEAlang
The Click SEAlang Firefox Add-on looks up words with a single click.  Results are sent to a standard panel (not a tab or new window).

    click here to install "Click SEAlang" beta (0.68a)
    Highlight a Thai, Khmer, Lao, or Burmese word, right-click, then choose "SEAlang search ... " (search dictionary) or "SEAlang tools ... " (search corpus or other tools) to find it.      see screenshots of Click SEAlang
    Highlight an English word, right-click, then choose "SEAlang tools ... Reverse-search ..." to reverse-search the SEAlang dictionary or bitext corpus.
    The default English reverse-search dictionary/corpus is Thai.  You can change the reverse search tool to Burmese, Lao, or Khmer after installing Click SEAlang by going to Tools | Add-ons | SEAlang ... Options in your browser.
    No change is needed for a regular search for Burmese, Lao, or Khmer in the dictionary, corpus, or bitext - only reverse search.
Download Error -228
This error may be caused by having the Firefox cache disabled.  To re-enable:
    Firefox 1.x Tools -> Options -> Privacy -> Cache
    Firefox 2.x Tools -> Options -> Advanced -> Network
Make sure the cache is at least 1MB. 
    For further information see this mozillaZine page.