Why are fonts rendered differently in WinXP/Win7?

Question

HTML

We have: the same versions of the browser (in my case, Firefox) under Win7 and WinXP.
I understand that rendering is different there, in Win7 it is subpixel, and in XP ClearType is most likely disabled (at least in my VirualBox it definitely does not turn on). But this maximum should affect anti-aliasing? Or all the same on the sizes of characters? Moreover, I believe that the same fonts may differ slightly in WinXP and Win7. Or a hint? But, okay...
The problem is that I also have a smaller line length in WinXP. As a result, the text, which in WinXP fits in one line in a limited block, in Win7 already fits in two, well, this is ... very bad in our case.
So. I began to test. letter-spacing, white-space - I fix everything in pixels.
Does not help.
Tried to use loadable OTF. And, strangely, the result is also not the same. But, if when using the system font, you can see the difference in the length of words (which, in total, gives a different line size), then when using your own font, the difference is mainly IN THE SIZES OF WORLD SPACING. Which, damn it, should be fixed for sure .... I
am attaching a picture on which there are two lines on top (the system font, and then my font) in the seven, and then two lines in XP (also the system font, and then my font) I

suspect that this is a dead end, and there are no solutions to come up with.
But, can anyone tell me the reasons for this behavior ...

Answer 1

Different graphics card drivers when acceleration is enabled in Firefox. Many factors affect smoothing.
In general, you should already score on retrogrades, let them enjoy their XP.