This question seems to come up every few days. The problem is, every time we answer it, those who miss those answers won't see it, because they seemingly disappear. So here is the answer posted as a sticky, so that it should remain visible forever and ever.
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The new board software defaults to show threads active in the last day.
To change all forums to the same setting, and to make the board "remember" your preference:
At the top of each page, you should see a menu bar. The leftmost item is labelled "User CP" (for User Control Panel). Click that. Then, on the page that is loaded, look for "Edit Options." Then, on the page that is loaded, look for "Default Thread Age Cut Off". Set this to your preference. If your preference is to see ALL threads, set it to "Show All Threads."