<tspan> with letter-spacing is rendered differently in firefox and inkscape

Description

So this simple svg is rendered differently in inkscape and firefox

<?xml version="1.0" encoding="UTF-8"?>
<svg version="1.1" width="350" height="150" viewBox="0 0 350 150" xmlns="http://www.w3.org/2000/svg">
    <text x="80" y="91.95" font-size="53.333px" font-family="Open Sans Condensed" fill="#808080">
		<tspan letter-spacing="0.05px">M</tspan>
		<tspan letter-spacing="0.005px">y</tspan>
		<tspan letter-spacing="0.05px">b</tspan>
		<tspan style="letter-spacing:0px">g</tspan>
		<tspan style="fill:#ff851b;letter-spacing:0.05px">Text</tspan>
	</text>
</svg>
Inkscape:

20251117_00h30m33s_grim

Firefox (145):

20251117_00h30m27s_grim

Thoughts

I can imagine that correct behavior is to apply letter-spacing only to corresponding tag for which it is defined although I crafted this svg using inkscape assuming that behavior of inkscape is correct which feels kinda natural. So I don't know if it is a bug of inkscape or a bug of firefox. Sorry for possibly false positive issue

Version Info

Inkscape 1.4.2 (ebf0e940d0, 2025-05-08)

                      Compile  (Run)
    GLib version:     2.84.4
    GTK version:      3.24.49 (3.24.49)
    glibmm version:   2.66.8
    gtkmm version:    3.24.10
    libxml2 version:  2.15.0
    libxslt version:  1.1.43
    Cairo version:    1.18.4 (1.18.4)
    Pango version:    1.56.4 (1.56.4)
    HarfBuzz version: 12.1.0 (12.1.0)

    OS version:       NixOS 25.11 (Xantusia)