SVG letter-spacing also applied to Mozilla Firefox
An alternative to letter-spacing that does work on Firefox is the textLength property. Perhaps that will suit you as a workaround?
<svg width="10cm" height="3cm" viewBox="0 0 1000 300"
xmlns="http://www.w3.org/2000/svg" version="1.1">
<desc>Example text01 - 'Hello, out there' in blue</desc>
<text x="250" y="150"
font-family="Verdana" font-size="55" fill="blue" >
Hello, out there
</text>
<text x="250" y="200" textLength="600"
font-family="Verdana" font-size="55" fill="blue" >
Hello, out there
</text>
<!-- Show outline of canvas using 'rect' element -->
<rect x="1" y="1" width="998" height="298"
fill="none" stroke="blue" stroke-width="2" />
</svg>You can use the parameter "dx".
<svg width="10cm" height="3cm" viewBox="0 0 1000 300"
xmlns="http://www.w3.org/2000/svg" version="1.1">
<text x="250" y="200" dx="0 20 20 20 20 0 20 20 20 20 20"
font-family="Verdana" font-size="55" fill="blue" >
Hello, out there
</text>
</svg>