Presently running Firefox ESR v45.4 and Pale Moon v26.5; both are OK.
Out of curiosity I've just tried Pale Moon v27, the portable version as I don't want to risk the present one. All of the GUI text, including Bookmarks etc., was blurry, too small and slightly malformed.
Tried portable Cyberfox - same result.
Installed Cyberfox, still the same.
I have no idea what's causing this. No tweaking in the browser is any good, except that Classic Theme Restorer (bundled with Cyberfox) did correct the tabs - until a restart.
Any ideas please?