Not a good interface.

Useless, no context menu, no unit detection, even no selected number detection... It's basically a separate app in your browser with very poor ergonomics

Very complete, so complete if you are looking for an light daily convertor for usual metric conversions.

While good for most conversions, it misses the most essential aspect of lengths in imperial, which is that feet are given in feet/inches (for example, a meter is just over 3 foot 3.37 inches). Unfortunately, this add-on gives the conversion only in pure decimal form, which is awkward and inconvenient. It gives a meter as 39.37 inches or 3.28 feet, neither of which are very useful. Likewise if you wanted to go in the other direction but were given something like 8ft 7in, you'd have to whip out a calculator just to figure out what you're supposed to put in for feet or inches, but at that point you may as well keep a conversion chart on hand and just do the math yourself.

Otherwise, this is perfectly functional and quite useful for very big or very small units, but base 12 units aren't really handled the way that they should be.

Most useful add on ive ever used, so handy and simple.

Excellent! It does it job very well. This is what I was looking for among many other similar addons. Firefox must fix the search engine for extensions relevance, so we can find specific extensions faster and easier. It took me some minutes to end up on this add-on.

When it comes to simplicity of the user interface and how thorough the unit conversion between metric and imperial units, this is the best extension I found so far, but Avast antivirus detected it as an unsafe extension and removed it. I do not know if it is false positive on the part of Avast but it certainly brings concern. I cannot, without hesitation, recommend it for that reason.

Excellent unit conversion extension. However, now I can't use it because of the new Firefox standards for extensions after updating from 56.0 to the newer Firefox. I can't even download it now. Nothin' like a hostile takeover of the base software for the sake of disruptive stringency.