I use ImTranslator. It's a browser add-on for firefox that does just that. You hold alt, select a word or phrase, and it pops up a dialog like this one.
Two extensions indisepnsible for anyone learning Japanese and/or trying to read Japanese text on websites:
Furiganaize (adds phonetic transcription for Kanji): https://addons.mozilla.org/en-US/firefox/addon/furiganaize/?src=search
ImTranslator (full featured translstion suite, including inline translation support): https://addons.mozilla.org/en-US/firefox/addon/imtranslator/
With these two extensions one can turn even complex Japanese text into something one can understand and lear from (phonetic transcription + inline translations).
As I'm writing this, my CPU is using about 3%-4% with 14 tabs open. About a week ago, I was getting CPU spikes to 12-14%. I opened up about:performance and noted an add-on at the top of the list was consuming massive amounts of CPU power. Since I removed it, my Firefox CPU usage is now in that normal 3%-4% range, except when loading tabs, etc., which is normal.
For those who are interested, this was the guilty add-on:
As an aside, I don't think this add-on was doing anything nefarious like Bitcoin mining. It is a bookmark tags add-on and I suspect it was poorly written. I noticed horrible CPU usage one time when I was reorganizing my bookmarks. That's when I decided to check out about:performance.
Also, I currently have this add-on usually at the top of my list for CPU usage:
Google Translate, ImTranslator, Dictionary, TTS
It has roughly 180k users. I wonder why an add-on that I use occasionally for translation is consuming CPU power even when I'm not asking it to do anything.
In any event, the about:performance feature is really helpful.
no. existing addons re-enable fine. reddit enhancement suite is working right now for example. the problem is clicking add to firefox on https://addons.mozilla.org/en-US/firefox/addon/imtranslator/?src=search does not work and results in the download error mentioned.
Using Google Translate, ImTranslator, Dictionary, TTS and after select text, it works for me when selecting imtranslator in the right-mouseclick popup window.