I obviously tried other fonts besides Noto Sans (Dejavu, Cantarell).
I also tried to use fc-cache --really-force
The font of the numbers and space is also very strange
Slightly related :
Thanks for the reply. I’ve tried but the result doesn’t change (and changing that systemwide setting makes the fonts a bit blurry anyway)
To be precise, I put “none” for character hinting
I found this :
.
Here @keybreak provided detailed explanation for this issue. This whole topic is worth a try.
It’s not that issue, i think…spacing that wide i have no clue except problems with font / specific de font rendering engine 
I felt there is something to do with rendering of the fonts as the last image is describing irregular spacing of the fonts.
Similar issue :
Might be worth a try: create a fresh profile and see if you still have the issue.
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In the aforementioned link, there was discussion that after removing specific fonts, the issue can be fixed…
I got this from Mozilla website:
.
https://support.mozilla.org/en-US/questions/931977
I made a few attempts
I added
<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "urn:fontconfig:fonts.dtd">
<fontconfig>
<match target="font">
<edit name="embeddedbitmap" mode="assign">
<bool>false</bool>
</edit>
</match>
</fontconfig>
in the file /home/salvatore/.config/fontconfig/20-no-embedded.conf and nothing has changed
I tried going to give
sudo ln -fs /usr/share/fontconfig/conf.avail/11-lcdfilter-default.conf /etc/fonts/conf.d/
And
fc-cache --force
But it did not work
I created a new user with all settings clean, and the fonts are still the same
The folder ~ / .fonts is not in my home, but I tried to look for the fonts named in the topic but they are not in my system
Hardware acceleration is not active in my firefox
PS: Thunderbird doesn’t have this problem. Which is weird. Obviously I tried to put the same fonts as in Thunderbird, but it hasn’t changed.
In any case, the problem is not that urgent. it’s very annoying but I can live with it
Weird question, but are you 100% sure that Noto fonts are actually installed? 
What’s your de?
Gimme a break! Even i can’t live with your problem 
They are installed, my DE (KDE Plasma) has the default Noto Sans fonts. Do I have to install Noto-sans-extra as well?
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Nope…
Take a screenshot of:
Settings → Appearance → Fonts
I also tried with full
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Looks good to me, but:
-
Fixed width should be set to Noto Mono, not Noto Sans
-
Try to disable Force font DPI see if it makes difference
Removing “force font DPI” made firefox shrink but fonts always have a strange space
By strange you mean as huge spaces as it were in op? 
Dang…
Yep. is it strange that only Firefox has them and not Thunderbird, in theory they are not “similar” programs?
I would suggest to start from a clean user profile, if you want really to find the problem and not just playing around Sherlock 
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