I have an Old Firefox Data file, have redownloaded Firefox, it wants me to access the backup file, but I can't select the whole file when browsing from the Firefox instructions

Please give me a simple step by step breakdown of how to solve this. I am AuDhD, and am trying to use digital technology to assist with my problems with executive function. Internet security requirements seem determined to stop this, by not consistently saving the data I expect them to, and then asking for it when it is too late for me to have realised I needed a 3rd party solution. I don't think I should need several products to achieve this very simple goal, but neither Google, Mozilla, Microsoft not Opera seem to agree This is now causing distress and taking a huge amount of time to try and resolve with companies that profit hugely from their customers, but refuse to talk to them directly.