Not working with Firefox 46.0.1 Rated 1 out of 5 stars

This doesn't seem to show any way to install a collection with Firefox 46.0.1.

This review is for a previous version of the add-on (2.0.1-signed.1-signed). 

Bad UI design on my part?

The add-on doesn't add anything to the collection page, if that's what you are looking for.
You need to go to a collection page then click on the add-on's button which is part of the browser UI.
I added a screenshot to the listing to make it more clear.

Thanks for the review though, it led me to a bug with multi-page collections. I've sent in a fix which is under review. In the meantime you can grab the latest version from "See complete version history" on the listing if you want.

In case you are looking to automate add-on installation, Extension Auto-Installer is probably a better choice.

Good but looks a little unfinished. Rated 4 out of 5 stars

This is overall a great addon. However, it has a number of shortcomings (listed here in order of importance) :

1. Addons in collection with no direct "add to firefox" link, but for example "continue to download" are skipped *without warning or notification whatsoever*. I'd expect a warning specifying that I need to install them manually or a list of addons that this extension can't install.
2. Only install addons from a collection *displayed on the page*. If the collection as multiple page (pagination), all the pages need to be done manually (see 3. below).
3. Works only with collection in current tab. An option for multiple collection in multiple tabs/multiple page from a single collection in multiple tabs would be nice.
4. No check all/none to select which addons to install.
5. Seems to install addons from collection even if they were already installed previously. Possibility to uncheck addons to avoid installing them, but it is tedious (see 4.).
6. The possibility to remove all addons in a collection would be really nice.

This review is for a previous version of the add-on (1.1.1-signed.1-signed). 

Thanks for the feedback!

When I made this add-on, I never expected it to have any active users --- It was designed to be used once or twice before it gets removed.
Maybe that is indeed how it is used, users just don't remember to remove it after they are done.

In any case, I didn't want to implement too many features since it would take me too much time.

Today, looking at the numbers this add-on has, I feel implementing some of the extra features mentioned in your review is more justified.
There is a new version in the works that will hopefully address issue number 1 to 4.

For 5, there is no robust way to check if an add-on is installed using the information exposed in the collections page. Install Collection would
have to grab all the xpi first, which is highly inefficient. It is a wont-fix until there is some change to AMO that facilitate this. I will provide a warning
about Install Collection re-installing all the add-ons that are already installed.
(for well-written add-ons, re-installing does nothing. It would be the same as an update; all the preference are still there)

6 is a wont-fix for the same reason as 5. However if there is more demand for this feature, it could be done. (inefficiently)

If you are interested in the latest development progress, you can visit the project's Github repository.
The update should be done before Firefox 31 stable release, however the the review time of said update is beyond my control.

Thank you!

Edit:
The update made it though review and is now live!