I understand that search bar position is not changeable by theming, it's a Thunderbird team's decision, but it irks me to see it take up so much premium space. It was the same with browsers, it took many years and iterations to get where we are now (tabs on top, no wasted space) and I think those lessons should be carried over.
> where we are now (tabs on top, no wasted space) Tabs at the top is wasted space, I much prefer my tabs on the side instead, as most web content is taller than it is wide, and I have a widescreen monitor. I understand the choice of tabs on top when 640x480 was the most common resolution, but for desktop usage today? Tabs on top seems like an outdated layout choice.
Vivaldi & Floorp offer this through being highly customizable but they tend to have cracks around the edges of their use for the same reason.
I was first introduced to this with a Chrome flag back in 2011 https://www.askvg.com/how-to-enable-new-compact-navigation-f... but they ended up backing out for various reasons (the largest of which was probably the specific design used a pop-down url bar which went over the page area, so could be spoofed).
In 2021 Safari became the largest browser I've seen roll this out as a 1st party feature to general users, but it faced some backlash https://www.zdnet.com/article/how-to-get-more-space-in-safar... I'm not a big fan of their particular styling choices but the layout was pretty decent.