AUR is unsupported - and officiall Arch only supports x86_64 so AUR is x86_64.
While some custom scripts from AUR may be buildable for ARM architecture this is entirely unsupported by the AUR script maintainer - unless the maintainer has added the architecture to the buildscript.
Friends,
Thank you for discussing the topic of installing Firefox ESR for ARM! 
But I’m newbie in Linux, and it’s hard for me to understand the complicated things are you discussing about.
But I still want to know if it is possible to install Firefox ESR. And if so, how.
Explain this to me in more detail, please.
I very need Firefox 115 ESR to ARM! 
I have not forgotten about you. This mourning I had to start the compile all over again with a device with more ram. So far it seems to be doing ok. I will post something when I know if it compiles or not. It is about 3/4 done.
Darksky, thank you so much for your attention to my problem
I am looking forward to your success 
It is not possible to install from repo - the package doesn’t exist.
Technically - it may be possible to compile - but you will have to do it yourself.
Even if it succeeds - there is still a need for someone to maintain it - and I don’t see that happen.
This is turning into a projrct. The PKGBUILD compiled an instrimental browser then clobbered it then turned around and is compiling an optimized browser. So still churning along…
Even if no one will support it, I will not update it and I will have enough of it for about 5 years
Not even safety updates? 
If there is no FF ESR support, I will have no choice but to refuse updates.
Speaking of… These packages has been built and compiled with libs on the unstable branch. I doubt that this firefox-esr will run on the stable branch. Your best bet is to switch to and upgrade on the unstable branch. It might work on the testing branch.
As I agree with what others has been saying here. I am not going to maintain this package in the future and I have not tested this package. This is as far as I am willing to go.
Install the firfox-esr and your language pack with:
sudo pacman -U package-name
md5sum:
8ceaa9e0f51794dfb0dbd21f458f7dc3 firefox-esr.tar
https://drive.google.com/file/d/1O26jQqok8NMICvQfghxzS2RIbopDesr-/view?usp=sharing
I don’t understand well how to install your Firefox ESR.
I tried make like this:
$ sudo pacman -U firefox
[sudo] password for user:
loading packages...
error: 'firefox': could not find or read package
Also tried:
$ sudo pacman -U firefox-esr-115.tar
loading packages...
error: missing package metadata in firefox-esr-115.tar
error: 'firefox-esr-115.tar': invalid or corrupted package
Could you specify in more detail the steps for installing it?
You really need to learn some very basic skills.
Download the tarball, unpack and install firefox-esr.
tar -xf firefox-esr.tar
cd firefox-esr
sudo pacman -U firefox-esr-115.4.0-1-aarch64.pkg.tar.zst
Install your language pack. I believe you might need Russian.
cd languages
sudo pacman -U firefox-esr-i18n-ru-115.4.0-1-aarch64.pkg.tar.zst
I do not know what branch you are so run from the terminal and see if firefox-esr will start up.
firefox-esr
If it does not start up and you see lib errors in the terminal then switch to the unstable branch then upgrade and reboot then try to run firefox-esr again.
sudo pacman-mirrors -aS testing && sudo pacman -Syyu
sudo reboot
After rebooting try running firefox-esr to see if it starts.
Utter nonsense, indeed. This post of yours is a clear example.
LLAP. 
Darksky: Oh, this is huge! You managed to make Firefox ESR 115!!!
Thank you so much for your great help!
Now I have a chance to switch from gluttonous Intel i3 to economical ARM S922x, which consumes much less power: 6 Wt vs. 60 Wt.
Why do you think that I need a Russian language pack?
I live in Ukraine, where heavy bombing and power outages are expected again this winter.
That’s why I need an economical computer that consumes little power and can long be powered by battery.
Dear friend, you helped me a lot with this, thank you very much!!! 
Someone mentioned about you posted in Russian. I see some of your overall reasoning now. Your welcome. Stay safe!!
Yes, I understand both Russian and Ukrainian languages.
Here is my funny workstation computer on ARM 
However, Manjaro works well and quickly on it.
But there are shortage with the necessary softwareю
You have already made the necessary Firefox, thank you very much.
Dear friend, I’m sorry for my impudence 
Could you do one more important favor?
To completely switch from Intel to ARM, I also need an Sylpheed email client.
It is too in AUR, but does not want to installed in Manjaro ARM.
Yes, I know that there is a Claws client, but he doesn’t suit me.
Could you also assemble a Sylpheed for the Manjaro ARM?
And then I could work with him too!
@Craft, I see you manage to get Manjaro working on Beelink GT King.
It is fairly easy to mogrify the PKGBUILD only a few commands are needed.
Our inhouse packager notified me that one should be able to skip the step modifying the PKGBUILD - using -A argument - thus this should do it
sudo pacman -Syu base-devel git micro
git clone https://aur.archlinux.org/sylpheed.git
cd sylpheed
makepkg -Asrcif
If you rather edit the PKGBUILD
The line reading
arch=(x86_64)
Change to and save the file
arch=(aarch64)
Then run
makepkg -is
It wasn’t me, it was done thanks to the Darksky
linux-aarhus,
Yes, I have already tried these steps, but as a result I got the error of missing aarch64 file
Update.
Ops, I am sorry, linux-aarhus!
It turns out I was taking the wrong steps! 
Now I have repeated your steps - and the Sylpheed has gathered and earned!
linux-aarhus, thank you so much!