The Bible in Living English (by Byington; copyright 1972 by the WT) in Hebrews 1:8 says "... God is your throne for ever and ever ..." and at Psalms 45:6 it says "God is your throne forever and evermore ...".
The Bible; A New Translation (by Moffatt; copyright 1935) in Hebrews 1:8 says "... God is thy throne for ever and ever ...". Oddly at Psalms 45:6 it doesn't use the word "God", but uses the phrase "shall stand" instead.. In that verse it says "Your throne shall stand for ever more ..." The reason for such could be the scholarly conjecture (#3) mentioned in the translators' note to Hebrews 1:8 in the Fifth Edition of the The New Testament In Modern Speech (by Weymouth and "Newly Revised By James Alexander Robertson") in reference to Psalms 45:6. That NT by Weymouth says the conjecture says the following.
"(3) A corrupt Hebrew text, ' Yahweh ' (God), being a mistake for the almost identical Hebrew word meaning ' shall be '--' Thy throne shall be for ever and ever.' This conjecture is widely adopted, but the writer of the Epistle, in applying the wors of the psalm to the Son, would not feel the difficulty ; and ' Thy throne, O God ' may stand."
Weymouth's NT says conjectures #1 through #2 say the following.
"(1) ' Thy throne is the throne of God ' (so. R.V. mg. in the Psalm).
(2) Thy throne is God for ever and ever.' "