Question: I have a question for you in the midst of my personal tribulation (I am hanging in there). Could you tell me why the Lord says that he will make his enemies a foot stool, and says elsewhere the earth is his foot stool (Hebrews 1:13 and Isaiah 66:1)?
Response: I believe what we have in both cases is are figurative uses. In Isaiah 66:1 it also says "heaven is My throne". On the one hand God exceeds the heavens and the earth by infinity (if He could even be described in these terms); on the other hand He actually has a localized throne in heaven (specifically, the third heaven) for the benefit of His creatures (Rev.4). But it is only in heaven - it is not technically speaking heaven itself. This passage is written in poetry, and this is typical poetic language - not wrong, but rather highly dramatic to make the point. If one were to rephrase this verse in prose it would be something like: "I rule in heaven; and I lord it over the earth". For that is what the throne and throne's footstool are really meant to convey, namely, powerful regal control - God's sovereign control of the heavens is such that it is as if they were His very throne, and His sovereign control over the earth is such that it is as if it were merely the place where He rests His feet.
This brings us to the Hebrews 1:13 (quoting another piece of Hebrew poetry, namely, Ps.110:1). In this second quote the same figure of the Lord's domination is in view. Just as God's control over the earth is so complete that it is just as if it were a footrest to Him, so the Messiah's domination and control over all His enemies will be made so complete on the day Jesus Christ returns to the earth (in the will and plan of the Father at the Second Advent) that it will be as if all who oppose Him were merely furniture beneath His feet. So, in my view, it is the same essential metaphor working in both cases; it is just that the object of the total domination is different in each case.
On a personal note, I am very encouraged to hear that you continue to persevere in your trials. It is only through many tribulations that we enter the kingdom of heaven (Acts 14:22). So there is nothing for it for those who love the Lord than to fight this fight till the fight is won and to run this race till the race is run.
Please see also the following links:
The Throne of God
The Session of Jesus Christ
The Coronation of Christ at the Second Advent
In Him who drank the cup right down to the dregs, our example and our Savior, our dear Lord Jesus Christ.