Box-drawing characters in Fira Sans don't join up

[sh1boot]

These box-drawing characters don't line up:

┌─┬─┬─┬─┬─┐
│ │ │ │ │ │

▛▀▜ ▛▀▜
▌ ▐ ▌ ▐
▙▄▟ ▙▄▟

𜸞𜸠𜸥     𜸣 𜸣       𜸦 𜸚𜸟𜸤𜸚𜸟𜸤 𜸣 𜸚𜸟𜸤
 𜸩𜸚𜸟𜸤𜸚𜸟𜸥𜸺𜸥𜸜𜸛𜸟𜸤𜸚𜸟𜸧 𜸩 𜸚𜸟𜹃 𜸟𜸷𜸪𜸺𜸸 𜸚𜹃
 𜸼𜸾𜸟𜸥𜸞𜸟𜹃𜸾𜸥𜹀𜸼 𜸼𜸾𜸟𜸶𜸞𜹀𜸥𜸽𜸟𜸥𜸾𜸟𜹃 𜸭  𜹍 
              𜸾𜸟𜹃

┌─┬─┬─┬─┬─┐
│ │ │ │ │ │

▛▀▜ ▛▀▜
▌ ▐ ▌ ▐
▙▄▟ ▙▄▟

𜸞𜸠𜸥     𜸣 𜸣       𜸦 𜸚𜸟𜸤𜸚𜸟𜸤 𜸣 𜸚𜸟𜸤
 𜸩𜸚𜸟𜸤𜸚𜸟𜸥𜸺𜸥𜸜𜸛𜸟𜸤𜸚𜸟𜸧 𜸩 𜸚𜸟𜹃 𜸟𜸷𜸪𜸺𜸸 𜸚𜹃
 𜸼𜸾𜸟𜸥𜸞𜸟𜹃𜸾𜸥𜹀𜸼 𜸼𜸾𜸟𜸶𜸞𜹀𜸥𜸽𜸟𜸥𜸾𜸟𜹃 𜸭  𜹍 
              𜸾𜸟𜹃 

Now you might call me silly for expecting a proportional font to support text-based box-drawing, but if there's no other constraint relating to proportional fonts, then shouldn't box-drawing characters all behave as-if they were monospaced and have the same width as some agreed baseline like figure space (which is the space I've used above to illustrate)?

But also, even Fira Mono doesn't successfully line up all of those.