Scaling font size not working - layout.css.devPixelsPerPx

[sevenstreets]

I am using Thunderbird 91.5.0 (latest).
OS: Linux Mint 20.3 (latest).

I am trying to increase the font size for everything by using the config editor to change layout.css.devPixelsPerPx to something like 1.2 or so. It scales fine and looks good. But when I close Thunderbird and open it again, it displays the default font size again (except in tabs like preferences etc, where the font is still larger). The setting for layout.css.devPixelsPerPx is what I set it to (1.2). If I reset it, everything gets super tiny. When I close and re-open thunderbird, it again returns to the default font size.
I am using a fresh install of Thunderbird and fresh profile. Before installing, I totally removed the ~/.thunderbird folder so no settings should carry over as far as I know.

Really appreciate the help on this forum.

[tanstaafl]

Check whether something prevents prefs.js from being updated. It sounds like the setting is stored only in memory, not on disk.

Prefs.js should include user_pref("layout.css.devPixelsPerPx", "1.2");

If the setting is saved in the file you might try help -> troubleshooting mode (aka safe mode) to temporarily disable all add-ons and userchrome.css, to see if they were overriding the settings effect.

[sevenstreets]

Hey tanstaafl, I gave that a try. I have confirmed that the pixelsperpx setting does store as it should, even after closing and restarting TB.
user_pref("layout.css.devPixelsPerPx", "1.7");

I have created a couple screenshots. It looks to me like graphic elements remain scaled after the restart, but all text elements go back to their default size. Looks like maybe a bug?
screenshot 1 - After changing Pixels value, but no restart -looks good-: https://drive.google.com/file/d/1sTfKBk ... sp=sharing
screenshot 2 - After restart (this one I scaled to 2.5 just to more clearly show what's happening): https://drive.google.com/file/d/1t2ZaWl ... sp=sharing

I also tried in troubleshooting mode. No change. This profile has no addons and no userChrome.css

[tanstaafl]

I tried setting it to 2.5. Usually I either use the defaults or set it to 1.5. Insane sizing on the folder pane but the icons and text were displayed okay. Some areas in tools -> preferences clipped the top half of the first sentence in a paragraph. When I reset the setting back to normal everything was fine, both the icons and the text were back to normal size. I can't duplicate your problem. Its probably very configuration specific or due to something getting corrupted.

Your second image seems to show the bottom half of icons in the folder pane is clipped but the other windows icons are okay. Since it appears to be a folder pane specific problem try both deleting foldertree.json and xulstore.json in the profile and restarting. It will create replacements using default values.

http://kb.mozillazine.org/Toolbar_or_wi ... ot_working

Try tools -> clear recent history. If that doesn't help try emptying the cache directory in the profile and restarting. There is also a -purgecaches command line argument though I think that's just to clear the javascript cache.