Spent a lot of my time just trying to get it to work. Finally I've done it but I'm an IT technician and I believe that most of regular people will fail here.
And according to the feedbacks it seems there is a lot of such failed tryouts when people can't even start to use it. Please do something with that.
For example my case was that:
1. Different versions (64-bit FireFox against x86 KeePassXC portable). Extension runs proxy but nothing happens futher - extension doesn't see proxy running or something like that. Fail.
2. KeePassXC (installed) doesn't make org.keepassxc.keepassxc_browser_firefox.json nor apropriate registry entry - proxy doesn't run. I believe that's because of UAC and permissions. Still hothing works. Fail again.
3. Portable x64 KeePassXC + Firefox x64 - win. It creates json and write proper registry key. Extension runs proxy and they both interact each other.

To follow up on (https://addons.mozilla.org/en-US/firefox/addon/keepassxc-browser/reviews/1279287/)
I also feel this extension has little to no use (especially for Firefox) First of all. the auto type sequence is a highly dangerous function. Even if the record inside the keepass database is all set to work for multi page logon, and one changes the global settings by accident. (or update!) all the custom fields are vanished Thus text is just injected into space.
And this is just a KeepassXC bug/~feature~
*sidenote: On Chromium on ubuntu the extension just works out of the box.
On firefox 10 out 10 times this extension is not connected to the database and even if the extension believes to be connected its not, due to KeepassXC not getting kicked alive.

For these hasles it it just not worth the effort.
Just learn the keyboard shortcuts to quickly find and copy the fields needed and insert them manually. The proxy to tunnel the uname/pwd into the browser will not be available i assume.
Second to that: I believe that if your pc is already in a state where you can't trust your computers (basic) functions as per example the clipboard; one has a whole lot of other problems to deal with prior to thinking about security and privacy enhancements
endnote: 5 (Five) stars for the time and effort from the team on KeepassXC itself

Works well for me! Just wished such an addon existed for Opera, too...

The issue that I have is that I don't understand what is the problem that this extension is trying solve. In order for it to work at all, you have to have KeePassXC already open and accessing the database that you are interested in. Other than this, modern browsers already do autofill of names and passwords that you have supplied, the latter being transparently encrypted by the browser itself. I am sure that there are legitimate uses for KeePassXC-browser but, as far as I am concerned, it is far more trouble than it is worth.

((URL detecting is just broken (when clicking plugin's icon it's just says it's connected to database and if activated through right click on username fiels -> KeePassXC-Browser -> "Fill Username and Password" the "Error: No logins found." notification pops up). It's firefox-specific bug, because url detecting and fields fill works fine in google chrome.))
UPD: fixed by disabling Enpass extansion. Please add note about other password managers' extensions to the FAQ. Also, multiple URLs support would be awesome.

Sometimes it's doesn't work, I must reinstall addon and it's work again.

Useful but lacks proper documentation. I could only find documentation pertaining to installation and configuration, not to actual usage.

The best one right now.

Not working in 99% of the time. You have to delete and and create database connection every single day because your current connection "Cannot decrypt message" or smth else.

Both the tool as well as the plugin works OOTB with Firefox 64 (Linux Mint/Ubuntu Bionic). Good work, I like it.

With Ubuntu 18.04 and latest Firefox it is unreliable, in 2/3 of the cases I get a Timeout and in 1/3 of the cases I'm able to connect. I couldn't find any reason why it works or why It doesn't work. I tried on two different machines. I couldn't find a way that reliably gets a connection. And I do have apparmor working. Trying it was a waste of time.

Was really looking forward to trying this out, but I can't seem to get it to work at all.

Tested with FF 64.0 Portable, KP XC 2.3.4 Portable, and followed directions at https://keepassxc.org/docs/keepassxc-browser-migration/ , but cannot seem to get the browser to connect to the DB. Per the instructions, I've enabled browser integration for FF, but receive the following Error from the plug-in: "Cannot encrypt message or public key not found. Is native messaging or support for your browser enabled in KeePassXC?". Reload button doesn't help, nor does restarting the apps.

EDIT:
Thanks for responding. It sounds as if I need to make some changes to the FF launcher's ini file (in order to get FF Portable to work with this), but it's unclear what those changes are. After searching around a bit more, I see that others are experiencing this problem as well:
https://portableapps.com/node/57907
https://github.com/keepassxreboot/keepassxc/issues/1817

I saw some information at https://github.com/keepassxreboot/keepassxc/issues/1559#issuecomment-369508900 regarding what registry entries need to be shared, but it's unclear to me exactly what steps I need to take to get FF Portable to work with KeePass-XC portable. Could you provide a bit more information about exactly what .ini files I need to edit and with what changes?

EDIT 2: Another user commented that disabling the proxy (Tools > Settings > Browser integration > Advanced) solved the integration issues. I've tried this, and it seems to be a good workaround. The downside is that a second copy of KeePass XC launches with the proxy disabled, but at least it seems to work.

with KeepasXC 2.3.4 (only this version) plug-in works as intended.
The only things missing now are:
- Ability to unlock database in the background (i.e. without opening KeePassXC
- Automatically re-scan login forms to detect / identify login fields.

== my original 1* review ==
Does not work. Every time I close my browser, I have to go though "connect" process.
Even if it did work - one has to manually open KeePassXC application and unlock database (which means, navigating away from the browser). If I'm opening KeePassXC anyway, why not copy/paste password from the application then, why use some plugin?

While the program works very well, the extension does not. Website detection is hit or miss, and there is a fill TOTP button that does absolutely nothing.